EMPLOYEE BENEFIT PLANS (Tables)
   | 
12 Months Ended | 
Dec. 31, 2019  | 
| Compensation and Retirement Disclosure [Abstract] | 
 
 | 
| Schedule of components of pension expense, the funded status of the plan, amounts recognized in the consolidated statements of financial condition, and major assumptions | 
| (Dollars in Thousands) | 
2019 | 
 | 
2018 | 
 | 
2017 | 
 
| Change in Projected Benefit Obligation: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Benefit Obligation at Beginning of Year | 
$ | 
149,347 | 
 | 
$ | 
165,084 | 
 | 
$ | 
152,585 | 
 
| Service Cost | 
 | 
6,114 | 
 | 
 | 
6,884 | 
 | 
 | 
6,752 | 
 
| Interest Cost | 
 | 
6,178 | 
 | 
 | 
5,661 | 
 | 
 | 
5,750 | 
 
| Actuarial Loss (Gain)  | 
 | 
25,715 | 
 | 
 | 
(16,349) | 
 | 
 | 
10,877 | 
 
| Benefits Paid | 
 | 
(6,255) | 
 | 
 | 
(11,686) | 
 | 
 | 
(10,541) | 
 
| Expenses Paid | 
 | 
(269) | 
 | 
 | 
(247) | 
 | 
 | 
(339) | 
 
| Projected Benefit Obligation at End of Year | 
$ | 
180,830 | 
 | 
$ | 
149,347 | 
 | 
$ | 
165,084 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Change in Plan Assets: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Fair Value of Plan Assets at Beginning of Year | 
$ | 
134,535 | 
 | 
$ | 
129,719 | 
 | 
$ | 
113,813 | 
 
| Actual Return (Loss)  on Plan Assets | 
 | 
28,635 | 
 | 
 | 
(6,251) | 
 | 
 | 
16,786 | 
 
| Employer Contributions | 
 | 
5,000 | 
 | 
 | 
23,000 | 
 | 
 | 
10,000 | 
 
| Benefits Paid | 
 | 
(6,255) | 
 | 
 | 
(11,686) | 
 | 
 | 
(10,541) | 
 
| Expenses Paid | 
 | 
(269) | 
 | 
 | 
(247) | 
 | 
 | 
(339) | 
 
| Fair Value of Plan Assets at End of Year | 
$ | 
161,646 | 
 | 
$ | 
134,535 | 
 | 
$ | 
129,719 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Funded Status of Plan and Accrued Liability Recognized at End of Year: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Other Liabilities | 
$ | 
19,184 | 
 | 
$ | 
14,812 | 
 | 
$ | 
35,365 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Accumulated Benefit Obligation at End of Year | 
$ | 
156,327 | 
 | 
$ | 
130,477 | 
 | 
$ | 
144,139 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Components of Net Periodic Benefit Costs: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Service Cost | 
$ | 
6,114 | 
 | 
$ | 
6,884 | 
 | 
$ | 
6,752 | 
 
| Interest Cost | 
 | 
6,178 | 
 | 
 | 
5,661 | 
 | 
 | 
5,750 | 
 
| Expected Return on Plan Assets | 
 | 
(9,527) | 
 | 
 | 
(9,564) | 
 | 
 | 
(8,026) | 
 
| Amortization of Prior Service Costs | 
 | 
15 | 
 | 
 | 
199 | 
 | 
 | 
223 | 
 
| Net Loss Amortization | 
 | 
3,862 | 
 | 
 | 
3,673 | 
 | 
 | 
3,812 | 
 
| Net Periodic Benefit Cost | 
$ | 
6,642 | 
 | 
$ | 
6,853 | 
 | 
$ | 
8,511 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Weighted-Average Assumptions Used to Determine Benefit Obligation: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Discount Rate | 
 | 
3.53% | 
 | 
 | 
4.43% | 
 | 
 | 
3.71% | 
 
| 
Rate of Compensation Increase(1)
 | 
 | 
4.00% | 
 | 
 | 
4.00% | 
 | 
 | 
3.25% | 
 
| Measurement Date | 
 | 
12/31/19 | 
 | 
 | 
12/31/18 | 
 | 
 | 
12/31/17 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Weighted-Average Assumptions Used to Determine Benefit Cost: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Discount Rate | 
 | 
4.43% | 
 | 
 | 
3.71% | 
 | 
 | 
4.21% | 
 
| Expected Return on Plan Assets | 
 | 
7.25% | 
 | 
 | 
7.25% | 
 | 
 | 
7.25% | 
 
| 
Rate of Compensation Increase(1)
 | 
 | 
4.00% | 
 | 
 | 
3.25% | 
 | 
 | 
3.25% | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Amortization Amounts from Accumulated Other Comprehensive Income: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Net Actuarial Loss (Gain)  | 
$ | 
6,606 | 
 | 
$ | 
(533) | 
 | 
$ | 
2,117 | 
 
| Prior Service Cost | 
 | 
(15) | 
 | 
 | 
(199) | 
 | 
 | 
(223) | 
 
| Net Loss | 
 | 
(3,862) | 
 | 
 | 
(3,673) | 
 | 
 | 
(3,812) | 
 
| Deferred Tax (Benefit) Expense  | 
 | 
(694) | 
 | 
 | 
1,118 | 
 | 
 | 
5,898 | 
 
| Other Comprehensive Loss (Gain), net of tax | 
$ | 
2,035 | 
 | 
$ | 
(3,287) | 
 | 
$ | 
3,980 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Amounts Recognized in Accumulated Other Comprehensive Income: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Net Actuarial Losses | 
$ | 
37,235 | 
 | 
$ | 
34,491 | 
 | 
$ | 
38,698 | 
 
| Prior Service Cost | 
 | 
50 | 
 | 
 | 
66 | 
 | 
 | 
265 | 
 
| Deferred Tax Benefit | 
 | 
(9,451) | 
 | 
 | 
(8,757) | 
 | 
 | 
(9,876) | 
 
| Accumulated Other Comprehensive Loss, net of tax | 
$ | 
27,834 | 
 | 
$ | 
25,800 | 
 | 
$ | 
29,087 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| 
(1) Effective 12/31/18, the Company utilized an age-graded approach that varies the rate based on the age of the participants.
 |  
  
 | 
| Schedule of pension plan asset allocation and the target asset allocation | 
 | 
 | 
 | 
Target | 
 | 
 | 
Percentage of Plan | 
 | 
 
 | 
 | 
Allocation | 
Assets at December 31(1)
 | 
 
 | 
 | 
 | 
2020 | 
 | 
 | 
2019 | 
 | 
 | 
2018 | 
 | 
 
| Equity Securities | 
 | 
 | 
65 | 
% | 
 | 
72 | 
% | 
 | 
67 | 
% | 
 
| Debt Securities | 
 | 
 | 
30 | 
% | 
 | 
19 | 
% | 
 | 
21 | 
% | 
 
| Cash and Cash Equivalents | 
 | 
 | 
5 | 
% | 
 | 
9 | 
% | 
 | 
12 | 
% | 
 
| Total | 
 | 
 | 
100 | 
% | 
 | 
100 | 
% | 
 | 
100 | 
% | 
 
  
(1)	Represents asset allocation at December 31 which may differ from the average target allocation for the year due to the year-end cash contribution to the plan. 
 
 | 
| Schedule of fair value of plan assets by level of the valuation inputs within the fair value hierarchy | 
| (Dollars in Thousands) | 
 | 
2019 | 
 | 
2018 | 
 
| Level 1: | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| U.S. Treasury Securities | 
 | 
$ | 
907 | 
 | 
$ | 
398 | 
 
| Mutual Funds | 
 | 
 | 
142,127 | 
 | 
$ | 
112,131 | 
 
| Cash and Cash Equivalents | 
 | 
 | 
13,943 | 
 | 
 | 
16,788 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Level 2: | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| U.S. Government Agency | 
 | 
 | 
2,078 | 
 | 
 | 
2,822 | 
 
| Corporate Notes/Bonds | 
 | 
 | 
2,591 | 
 | 
 | 
2,396 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Total Fair Value of Plan Assets | 
 | 
$ | 
161,646 | 
 | 
$ | 
134,535 | 
 
  
 | 
| Schedule of expected benefit payments related to the defined benefit pension plan | 
| 
Expected Benefit Payments.  At December 31, expected benefit payments related to the defined benefit pension plan were as follows:
 |  
 | 
 | 
 | 
 
| (Dollars in Thousands) | 
2019 | 
 
| 2020 | 
$ | 
14,230 | 
 
| 2021 | 
 | 
13,551 | 
 
| 2022 | 
 | 
13,268 | 
 
| 2023 | 
 | 
13,089 | 
 
| 2024 | 
 | 
13,216 | 
 
| 2025 through 2029 | 
 | 
57,999 | 
 
| Total | 
$ | 
125,353 | 
 
  
 | 
| Schedule of amounts contributed to the pension plan and the expected amount to be contributed | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
Expected | 
 
 | 
 | 
 | 
 | 
 | 
 | 
Contribution | 
 
| (Dollars in Thousands) | 
 | 
2018 | 
 | 
 | 
2019 | 
2020(1)
 | 
 
| Actual Contributions | 
 | 
$ | 
23,000 | 
 | 
$ | 
5,000 | 
 | 
$ | 
5,000 | 
 
  
 | 
| Schedule of components of SERP's periodic benefit cost, the funded status of the plan, amounts recognized in the consolidated statements of financial condition, and major assumptions | 
| (Dollars in Thousands) | 
2019 | 
 | 
2018 | 
 | 
2017 | 
 
| Change in Projected Benefit Obligation: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Benefit Obligation at Beginning of Year | 
$ | 
8,860 | 
 | 
$ | 
7,285 | 
 | 
$ | 
5,741 | 
 
| Interest Cost | 
 | 
349 | 
 | 
 | 
227 | 
 | 
 | 
191 | 
 
| Actuarial Loss | 
 | 
1,035 | 
 | 
 | 
1,348 | 
 | 
 | 
1,353 | 
 
| Projected Benefit Obligation at End of Year | 
$ | 
10,244 | 
 | 
$ | 
8,860 | 
 | 
$ | 
7,285 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Funded Status of Plan and Accrued Liability Recognized at End of Year: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Other Liabilities | 
$ | 
10,244 | 
 | 
$ | 
8,860 | 
 | 
$ | 
7,285 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Accumulated Benefit Obligation at End of Year | 
$ | 
8,778 | 
 | 
$ | 
7,557 | 
 | 
$ | 
6,485 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Components of Net Periodic Benefit Costs: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Interest Cost | 
 | 
349 | 
 | 
 | 
227 | 
 | 
 | 
191 | 
 
| Net Loss Amortization | 
 | 
761 | 
 | 
 | 
1,626 | 
 | 
 | 
597 | 
 
| Net Periodic Benefit Cost | 
$ | 
1,110 | 
 | 
$ | 
1,853 | 
 | 
$ | 
788 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Weighted-Average Assumptions Used to Determine Benefit Obligation: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Discount Rate | 
 | 
3.16% | 
 | 
 | 
4.23% | 
 | 
 | 
3.53% | 
 
| 
Rate of Compensation Increase(1)
 | 
 | 
4.00% | 
 | 
 | 
4.00% | 
 | 
 | 
3.25% | 
 
| Measurement Date | 
 | 
12/31/19 | 
 | 
 | 
12/31/18 | 
 | 
 | 
12/31/17 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Weighted-Average Assumptions Used to Determine Benefit Cost: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Discount Rate | 
 | 
4.23% | 
 | 
 | 
3.53% | 
 | 
 | 
3.92% | 
 
| 
Rate of Compensation Increase(1)
 | 
 | 
3.50% | 
 | 
 | 
3.25% | 
 | 
 | 
3.25% | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Amortization Amounts from Accumulated Other Comprehensive Income: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Net Actuarial Loss | 
$ | 
1,035 | 
 | 
$ | 
1,348 | 
 | 
$ | 
1,353 | 
 
| Net Loss | 
 | 
(761) | 
 | 
 | 
(1,626) | 
 | 
 | 
(597) | 
 
| Deferred Tax (Benefit) Expense  | 
 | 
(70) | 
 | 
 | 
71 | 
 | 
 | 
(77) | 
 
| Other Comprehensive Loss (Gain), net of tax | 
$ | 
204 | 
 | 
$ | 
(207) | 
 | 
$ | 
679 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Amounts Recognized in Accumulated Other Comprehensive Income: | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Net Actuarial Loss | 
$ | 
1,622 | 
 | 
$ | 
1,348 | 
 | 
$ | 
1,626 | 
 
| Deferred Tax Benefit | 
 | 
(411) | 
 | 
 | 
(341) | 
 | 
 | 
(412) | 
 
| Accumulated Other Comprehensive Loss, net of tax | 
$ | 
1,211 | 
 | 
$ | 
1,007 | 
 | 
$ | 
1,214 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
| 
(1) Effective 12/31/2018, the Company utilized an age-graded approach that varies the rate based on the age of the participants.
 |  
  
 | 
| Schedule of expected benefit payments related to the SERP | 
| (Dollars in Thousands) | 
 | 
2019 | 
 
| 2020 | 
 | 
$ | 
4,059 | 
 
| 2021 | 
 | 
 | 
2,618 | 
 
| 2022 | 
 | 
 | 
2,478 | 
 
| 2023 | 
 | 
 | 
1,419 | 
 
| 2024 | 
 | 
 | 
211 | 
 
| 2025 through 2029 | 
 | 
 | 
- | 
 
| Total | 
 | 
$ | 
10,785 | 
 
  
 |