ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), Before and After Tax Amounts by Component (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Change in unrealized holding gain or loss on securities available-for-sale [Abstract]
Change in net unrealized holding gain or loss during the period	$ 3,793		$ 4,204		$ 14,053	$ 8,815
Reclassification adjustment for net (gains) losses included in net gain on securities and other assets	(3,134)		57		(3,142)	133
Net change	659		4,261		10,911	8,948
Tax expense	248		1,371		3,456	2,878
Net change in unrealized holding gain or loss on securities available-for-sale	411		2,890		7,455	6,070
Change in pension and other postretirement obligations [Abstract]
Reclassification adjustment for expense included in other expense	95		181		190	364
Change in the net actuarial gain or loss	176		56		352	365
Net change	271		237		542	729
Tax expense	85		76		171	234
Net change in pension and other postretirement obligations	186		161		371	495
Change in unrealized gain or loss on derivatives [Abstract]
Change in net unrealized gain or loss during the period	(4,617)		(5,745)		(25,198)	(8,673)
Reclassification adjustment for expense included in interest expense	1,156		(332)		1,360	(688)
Net change	(3,461)		(6,077)		(23,838)	(9,361)
Tax benefit	(1,093)		(1,970)		(7,549)	(3,008)
Net change in unrealized gain or loss on derivatives	(2,368)		(4,107)		(16,289)	(6,353)
Other comprehensive income (loss), net of tax	$ (1,771)	$ (6,692)	$ (1,056)	$ 1,268	$ (8,463)	$ 212