BENEFIT PLANS, Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2009
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	[1]	$ 171,625
Fair value of plan assets at end of year	[1]	180,086	$ 171,625
Amounts recognized in accumulated other comprehensive income (loss) (AOCI) [Abstract]
AOCI at year end		17,961	21,283	$ 16,716
Accumulated other comprehensive income (loss), negative postretirement medical plan amendment, arising during period, net of tax					$ 3,500
Pension Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year		127,134	127,513
Service cost		4,106	3,324	3,530
Interest cost		5,624	5,506	5,271
Plan amendment / acquisition		22	2,395
Participant contributions		0	0
Deferred actuarial (gain)/loss		(1,628)	(2,091)
Benefits paid		(8,174)	(9,513)
Benefit obligation at end of year		127,084	127,134	127,513
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		172,026	177,865
Actual return of plan assets		14,402	1,125
Participant contributions		0	0
Employer contributions		2,146	616
Plan acquisition		0	1,933
Benefits paid		(8,174)	(9,513)
Fair value of plan assets at end of year		180,400	172,026	177,865
Over/(Under) funded status at year end		53,316	44,892
Amounts recognized in the consolidated statement of condition [Abstract]
Other assets		64,709	56,361
Other liabilities		(11,393)	(11,469)
Amounts recognized in accumulated other comprehensive income (loss) (AOCI) [Abstract]
Net loss		28,323	34,016
Net prior service cost (credit)		2,264	2,307
Pre-tax AOCI		30,587	36,323
Taxes		(11,622)	(13,815)
AOCI at year end		18,965	22,508
Post-retirement Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year		1,918	2,256
Service cost		0	0	0
Interest cost		82	87	102
Plan amendment / acquisition		0	0
Participant contributions		516	509
Deferred actuarial (gain)/loss		174	(45)
Benefits paid		(884)	(889)
Benefit obligation at end of year		1,806	1,918	2,256
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		516	509
Employer contributions		368	380
Plan acquisition		0	0
Benefits paid		(884)	(889)
Fair value of plan assets at end of year		0	0	$ 0
Over/(Under) funded status at year end		(1,806)	(1,918)
Amounts recognized in the consolidated statement of condition [Abstract]
Other assets		0	0
Other liabilities		(1,806)	(1,918)
Amounts recognized in accumulated other comprehensive income (loss) (AOCI) [Abstract]
Net loss		183	4
Net prior service cost (credit)		(1,801)	(1,980)
Pre-tax AOCI		(1,618)	(1,976)
Taxes		614	751
AOCI at year end		(1,004)	(1,225)
Unfunded Supplementary Pension Plans [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year		11,400
Benefit obligation at end of year		11,300	11,400
Amounts recognized in accumulated other comprehensive income (loss) (AOCI) [Abstract]
Benefit obligation for defined benefit pension plan prior to plan revaluation		115,700	115,700
Unfunded Stock Balance [Member] | Nonemployee Directors [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year		100
Benefit obligation at end of year		$ 100	$ 100

[1] Excludes dividends and interest receivable totaling $0.3 million and $0.4 million at December 31, 2016 and 2015, respectively.