ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Before and After Tax Amounts by Component (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Change in unrealized holding gain or loss on securities available-for-sale:
Change in net unrealized gain or loss during the period	$ 8,966		$ 3,793		$ (6,568)	$ 14,053
Reclassification adjustment for net gains included in net gain on securities and other assets	(20)		(3,134)		(1,207)	(3,142)
Net change	8,946		659		(7,775)	10,911
Tax expense (benefit)	2,817		248		(2,512)	3,456
Net change in unrealized holding gain or loss on securities available-for-sale, net of reclassification adjustments and tax	6,129		411		(5,263)	7,455
Change in pension and other postretirement obligations:
Reclassification adjustment for expense included in other expense	(438)		95		(860)	190
Reclassification adjustment for curtailment loss					1,543
Change in the net actuarial gain or loss	644		176		1,529	352
Net change	206		271		2,212	542
Tax expense	95		85		730	171
Net change in pension and other postretirement obligations	111		186		1,482	371
Change in unrealized gain or loss on derivatives:
Change in net unrealized gain or loss during the period	(1,406)		(4,617)		3,542	(25,198)
Reclassification adjustment for loss included in loss on termination of derivatives					16,505
Reclassification adjustment for expense included in interest expense	10		1,156		864	1,360
Net change	(1,396)		(3,461)		20,911	(23,838)
Tax expense (benefit)	799		(1,093)		6,630	(7,549)
Net change in unrealized gain or loss on derivatives, net of reclassification adjustments and tax	(2,195)		(2,368)		14,281	(16,289)
Total other comprehensive income	$ 4,045	$ 6,455	$ (1,771)	$ (6,692)	$ 10,500	$ (8,463)