ACCUMULATED OTHER COMPREHENSIVE LOSS - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities available for sale	$ 0	$ (75)	$ (12)	$ (983)
Tax effect	1,710	1,209	4,250	3,130
Pension and other employee benefits	(992)	(1,573)	(3,417)	(4,894)
Net of tax	(3,654)	(2,745)	(9,589)	(7,073)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net of tax	21	186	54	86
Unrealized Gains and Losses on Securities Available for Sale | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities available for sale	0	(75)	(12)	(983)
Tax effect	0	28	4	371
Net of tax	0	(47)	(8)	(612)
Defined Benefit and Other Benefit Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Tax effect	(12)	(141)	37	423
Net of tax	21	233	62	698
Prior Service Costs | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Pension and other employee benefits	(55)	(22)	(165)	(67)
Actuarial Losses | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Pension and other employee benefits	$ (88)	$ (396)	$ (264)	$ (1,188)