ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Before and After Tax Amounts by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Change in unrealized holding gain or loss on securities available-for-sale:
Change in net unrealized gain or loss during the period	$ (15,534)	$ 10,260
Reclassification adjustment for net (gains) losses included in net gain on securities and other assets	(1,187)	(8)
Net change	(16,721)	10,252
Tax expense	(5,329)	3,208
Net change in unrealized holding gain or loss on securities available-for-sale	(11,392)	7,044
Change in pension and other postretirement obligations:
Reclassification adjustment for expense included in other expense	(422)	95
Reclassification adjustment for curtailment loss	1,543
Change in the net actuarial gain or loss	885	176
Net change	2,006	271
Tax expense	635	86
Net change in pension and other postretirement obligations	1,371	185
Change in unrealized gain or loss on derivatives:
Change in net unrealized gain or loss during the period	21,453	(20,581)
Reclassification adjustment for expense included in interest expense	854	204
Net change	22,307	(20,377)
Tax benefit	5,831	(6,456)
Net change in unrealized gain or loss on derivatives	16,476	(13,921)
Other comprehensive income (loss), net of tax	$ 6,455	$ (6,692)