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BENEFIT PLANS, Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2009 | May 12, 2017
Change in plan assets [Roll Forward] |  |  |  |  |  |
Fair value of plan assets at beginning of year | [1] | $ 180,086 |  |  |  |
Fair value of plan assets at end of year | [1] | 216,476 | $ 180,086 |  |  |
Amounts recognized in accumulated other comprehensive income (loss) ("AOCI") [Abstract] |  |  |  |  |  |
AOCI at year end |  | $ 21,633 | 17,961 | $ 21,283 |  |
Accumulated other comprehensive income (loss), negative postretirement medical plan amendment, arising during period, net of tax |  |  |  |  | $ 3,500 |
Stock Balance Plan [Member] | Nonemployee Directors [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Funding Status [Extensible List] |  | us-gaap:UnfundedPlanMember |  |  |  |
Change in benefit obligation [Roll Forward] |  |  |  |  |  |
Benefit obligation at the beginning of year |  | $ 100 |  |  |  |
Benefit obligation at end of year |  | 100 | 100 |  |  |
Pension Benefits [Member] |  |  |  |  |  |
Change in benefit obligation [Roll Forward] |  |  |  |  |  |
Benefit obligation at the beginning of year |  | 127,084 | 127,134 |  |  |
Service cost |  | 4,181 | 4,106 | 3,324 |  |
Interest cost |  | 5,717 | 5,624 | 5,506 |  |
Plan amendment / acquisition |  | 11,646 | 22 |  |  |
Participant contributions |  | 0 | 0 |  |  |
Deferred actuarial loss/(gain) |  | 8,978 | (1,628) |  |  |
Benefits paid |  | (10,156) | (8,174) |  |  |
Benefit obligation at end of year |  | 147,450 | 127,084 | 127,134 |  |
Change in plan assets [Roll Forward] |  |  |  |  |  |
Fair value of plan assets at beginning of year |  | 180,400 | 172,026 |  |  |
Actual return of plan assets |  | 22,954 | 14,402 |  |  |
Participant contributions |  | 0 | 0 |  |  |
Employer contributions |  | 9,839 | 2,146 |  |  |
Plan acquisition |  | 14,070 | 0 |  |  |
Benefits paid |  | (10,156) | (8,174) |  |  |
Fair value of plan assets at end of year |  | 217,107 | 180,400 | 172,026 |  |
Over/(Under) funded status at year end |  | 69,657 | 53,316 |  |  |
Amounts recognized in the consolidated statement of condition [Abstract] |  |  |  |  |  |
Other assets |  | 81,000 | 64,709 |  |  |
Other liabilities |  | (11,343) | (11,393) |  |  |
Amounts recognized in accumulated other comprehensive income (loss) ("AOCI") [Abstract] |  |  |  |  |  |
Net loss |  | 26,935 | 28,323 |  |  |
Net prior service cost (credit) |  | 2,944 | 2,264 |  |  |
Pre-tax AOCI |  | 29,879 | 30,587 |  |  |
Taxes |  | (7,340) | (11,622) |  |  |
AOCI at year end |  | $ 22,539 | $ 18,965 |  |  |
Unamortized actuarial gain |  |  |  |  |  | $ 1,900
Discount rate |  | 4.00% | 4.50% |  |  | 4.40%
Post-retirement Benefits [Member] |  |  |  |  |  |
Change in benefit obligation [Roll Forward] |  |  |  |  |  |
Benefit obligation at the beginning of year |  | $ 1,806 | $ 1,918 |  |  |
Service cost |  | 0 | 0 | 0 |  |
Interest cost |  | 76 | 82 | 87 |  |
Plan amendment / acquisition |  | 0 | 0 |  |  |
Participant contributions |  | 467 | 516 |  |  |
Deferred actuarial loss/(gain) |  | 245 | 174 |  |  |
Benefits paid |  | (809) | (884) |  |  |
Benefit obligation at end of year |  | 1,785 | 1,806 | 1,918 |  |
Change in plan assets [Roll Forward] |  |  |  |  |  |
Fair value of plan assets at beginning of year |  | 0 | 0 |  |  |
Actual return of plan assets |  | 0 | 0 |  |  |
Participant contributions |  | 467 | 516 |  |  |
Employer contributions |  | 342 | 368 |  |  |
Plan acquisition |  | 0 | 0 |  |  |
Benefits paid |  | (809) | (884) |  |  |
Fair value of plan assets at end of year |  | 0 | 0 | $ 0 |  |
Over/(Under) funded status at year end |  | (1,785) | (1,806) |  |  |
Amounts recognized in the consolidated statement of condition [Abstract] |  |  |  |  |  |
Other assets |  | 0 | 0 |  |  |
Other liabilities |  | (1,785) | (1,806) |  |  |
Amounts recognized in accumulated other comprehensive income (loss) ("AOCI") [Abstract] |  |  |  |  |  |
Net loss |  | 420 | 183 |  |  |
Net prior service cost (credit) |  | (1,622) | (1,801) |  |  |
Pre-tax AOCI |  | (1,202) | (1,618) |  |  |
Taxes |  | 296 | 614 |  |  |
AOCI at year end |  | $ (906) | $ (1,004) |  |  |
Discount rate |  | 4.00% | 4.40% |  |  |
Supplemental Pension Plans [Member] | Key Executives [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Funding Status [Extensible List] |  | us-gaap:UnfundedPlanMember |  |  |  |
Change in benefit obligation [Roll Forward] |  |  |  |  |  |
Benefit obligation at the beginning of year |  | $ 11,300 |  |  |  |
Benefit obligation at end of year |  | 11,300 | $ 11,300 |  |  |
Amounts recognized in accumulated other comprehensive income (loss) ("AOCI") [Abstract] |  |  |  |  |  |
Benefit obligation for defined benefit pension plan prior to plan revaluation |  | $ 136,100 | $ 115,700 |  |  |
[1] | Excludes dividends and interest receivable totaling $0.6 million and $0.3 million at December 31, 2017 and 2016, respectively.

X
- Definition The tax on net gain (loss) on prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. + References No definition available. + Details
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