BENEFIT PLANS, Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2009	Jun. 01, 2018	May 12, 2017
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
AOCI at year end	$ 32,837	$ 21,633	$ 17,961
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]
Pension Plan [Abstract]
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	147,450	127,084
Service cost	4,561	4,181	4,106
Interest cost	5,676	5,717	5,624
Plan amendment / acquisition	883	11,646
Participant contributions	0	0
Deferred actuarial (gain)/loss	(4,177)	8,978
Benefits paid	(10,182)	(10,156)
Benefit obligation at end of year	144,211	147,450	127,084
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	217,107	180,400
Actual return of plan assets	(3,858)	22,954
Participant contributions	0	0
Employer contributions	605	9,839
Plan acquisition	0	14,070
Benefits paid	(10,182)	(10,156)
Fair value of plan assets at end of year	203,672	217,107	$ 180,400
Over/(Under) funded status at year end	59,461	69,657
Amounts recognized in the consolidated statement of condition [Abstract]
Other assets	72,659	81,000
Other liabilities	(13,198)	(11,343)
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Net loss	39,410	26,935
Net prior service cost (credit)	4,939	2,944
Pre-tax AOCI	44,349	29,879
Taxes	(10,870)	(7,340)
AOCI at year end	$ 33,479	$ 22,539
Unamortized actuarial gain						$ 1,900
Discount rate	4.50%	4.00%	4.50%			4.40%
Pension Benefits [Member] | Restoration Plan [Member] | Nonqualified Plan [Member] | us-gaap_UnfundedPlanMember
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Initial projected benefit obligation					$ 800
Post-retirement Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year	$ 1,785	$ 1,806
Service cost	0	0	$ 0
Interest cost	69	76	82
Plan amendment / acquisition	0	0
Participant contributions	479	467
Deferred actuarial (gain)/loss	191	245
Benefits paid	(867)	(809)
Benefit obligation at end of year	1,657	1,785	1,806
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	479	467
Employer contributions	388	342
Plan acquisition	0	0
Benefits paid	(867)	(809)
Fair value of plan assets at end of year	0	0	$ 0
Over/(Under) funded status at year end	(1,657)	(1,785)
Amounts recognized in the consolidated statement of condition [Abstract]
Other assets	0	0
Other liabilities	(1,657)	(1,785)
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Net loss	592	420
Net prior service cost (credit)	(1,444)	(1,622)
Pre-tax AOCI	(852)	(1,202)
Taxes	210	296
AOCI at year end	$ (642)	$ (906)
Discount rate	4.45%	4.00%
Supplemental Pension Plans [Member] | Key Executives [Member]
Pension Plan [Abstract]
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	13,200	$ 11,300
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Benefit obligation for defined benefit pension plan prior to plan revaluation	$ 131,000	$ 136,100