ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Before and After Tax Amounts by Component (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Change in unrealized holding gain or loss on securities available-for-sale:
Change in net unrealized gain or loss during the period	$ (8,654)			$ 1,716			$ (15,222)	$ 15,769
Reclassification adjustment for net gains included in net gain on securities and other assets				(215)			(1,207)	(3,357)
Net change	(8,654)			1,501			(16,429)	12,412
Tax (benefit) expense	(2,714)			475			(5,226)	3,931
Net change in unrealized holding gain or loss on securities available-for-sale, net of reclassification adjustments and tax	(5,940)			1,026			(11,203)	8,481
Change in pension and other postretirement obligations:
Reclassification adjustment for expense included in other expense	(735)			4			(1,595)	194
Reclassification adjustment for curtailment loss							1,543
Change in the net actuarial gain or loss	941			267			2,470	619
Net change	206			271			2,418	813
Tax expense	65			86			795	257
Net change in pension and other postretirement obligations	141			185			1,623	556
Change in unrealized gain or loss on derivatives:
Change in net unrealized gain or loss during the period	225			100			3,767	(25,098)
Reclassification adjustment for loss included in loss on termination of derivatives							16,505
Reclassification adjustment for expense included in interest expense	38			2,319			902	3,679
Net change	263			2,419			21,174	(21,419)
Tax expense (benefit)	82			766			6,712	(6,783)
Net change in unrealized gain or loss on derivatives, net of reclassification adjustments and tax	181			1,653			14,462	(14,636)
Total other comprehensive (loss) income, net of tax	$ (5,618)	$ 4,045	$ 6,455	$ 2,864	$ (1,771)	$ (6,692)	$ 4,882	$ (5,599)