ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Before and After Tax Amounts by Component (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Change in unrealized gain (loss) on securities:
Change in net unrealized loss during the period	$ (4,374)			$ (39,692)			$ (17,967)	$ (142,980)
Reclassification adjustment for net losses included in net loss on sale of securities and other assets							1,447
Accretion of net unrealized loss on securities transferred to held-to-maturity	788			1,112			2,341	2,111
Net change	(3,586)			(38,580)			(14,179)	(140,869)
Tax benefit	(1,062)			(12,133)			(1,507)	(44,304)
Net change in unrealized loss on securities, net of reclassification adjustments and tax	(2,524)			(26,447)			(12,672)	(96,565)
Change in pension and other postretirement obligations:
Reclassification adjustment for expense included in other expense	(370)			(935)			(1,110)	(2,805)
Change in the net actuarial gain	518			997			1,553	2,992
Net change	148			62			443	187
Tax expense	185			20			272	59
Net change in pension and other postretirement obligations	(37)			42			171	128
Change in unrealized gain (loss) on derivatives:
Change in net unrealized (loss) gain during the period	(246)			5,387			(771)	14,548
Reclassification adjustment for expense included in interest expense	293			(546)			279	(569)
Net change	47			4,841			(492)	13,979
Tax expense (benefit)	14			1,522			(459)	4,397
Net change in unrealized gain (loss) on derivatives, net of reclassification adjustments and tax	33			3,319			(33)	9,582
Total other comprehensive loss, net of tax	$ (2,528)	$ (5,747)	$ (4,259)	$ (23,086)	$ (20,570)	$ (43,199)	$ (12,534)	$ (86,855)