CHANGE IN ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions		$ 0	$ 736	$ 21	$ 1,764
Income taxes		(9,893)	(10,942)	(27,569)	(30,382)
Income before taxes		48,828	31,511	132,594	90,458
Net income		38,935	20,569	105,025	60,076
Prior service cost (credit), tax adjustment	$ 100
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income		(144)	(596)	(375)	(1,500)
Reclassification out of Accumulated Other Comprehensive Income | Net change in defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service costs (credits)		(19)	(19)	21	(57)
Actuarial gains		(11)	(18)	(34)	(52)
Income taxes		(7)	15	(3)	42
Income before taxes		(30)	(37)	(13)	(109)
Net income		(37)	(22)	(16)	(67)
Reclassification out of Accumulated Other Comprehensive Income | Realized Gains on Securities Transactions | Net change in investment securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions		0	(736)	(21)	(1,764)
Income taxes		0	261	5	628
Net income		0	(475)	(16)	(1,136)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Net Unrealized Gains to Income During Period | Net change in investment securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net unrealized gains to income during the period		(142)	(159)	(451)	(478)
Income taxes		35	60	108	181
Net income		$ (107)	$ (99)	$ (343)	$ (297)