ACCUMULATED OTHER COMPREHENSIVE LOSS - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrealized gains and losses										$ 1,197	$ 816	$ 1,244
Tax benefit (expense)		$ (4,265)	$ (3,783)	$ (2,809)	$ (479)	$ (2,854)	$ (3,661)	$ (3,569)	$ (3,137)	(11,336)	(13,221)	(10,163)
Realized net (loss) gain on sale of securities		$ (24)	$ 78	$ 2,662	$ 5,541	$ 42	$ 42	$ 416	$ 256	8,257	756	(1,839)
Total reclassifications for the period, net of tax										69	(313)	(3,048)
Unrealized gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income (loss)										7,060	(60)	(3,083)
Realized net gain (loss) on interest rate swap derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income (loss)										(3,945)	2,043	1,406
Net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income (loss)	[1]									(3,035)	(2,386)	(2,189)
Prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income (loss)	[1]									7	8	7
Amortization of pension plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income (loss)										(3,028)	(2,378)	(2,182)
Tax benefit										636	499	459
Total reclassifications for the period, net of tax										(2,392)	(1,879)	(1,723)
Unrealized losses on securities transferred | Reclassification out of accumulated other comprehensive income | Unrealized gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrealized gains and losses	[2]									(1,197)	(816)	(1,244)
Tax benefit (expense)										251	171	261
Net of tax										(946)	(645)	(983)
Unrealized gains and losses on available for sale securities | Reclassification out of accumulated other comprehensive income | Unrealized gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)										(1,734)	(159)	386
Net of tax										6,523	597	(1,453)
Realized net (loss) gain on sale of securities	[3]									8,257	756	(1,839)
Interest rate swap derivatives | Reclassification out of accumulated other comprehensive income | Realized net gain (loss) on interest rate swap derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)										834	(411)	(277)
Net of tax										(3,136)	1,545	1,042
Realized net gain (loss) on interest rate swaps	[4]									(3,970)	1,956	1,319
Interest rate swap derivatives | Reclassification out of accumulated other comprehensive income | Amortization of unrealized gains on terminated interest rate swap derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)										(5)	(18)	(18)
Net of tax										20	69	69
Realized net gain (loss) on interest rate swaps	[4]									$ 25	$ 87	$ 87

[1] These AOCI components are included in the computation of net periodic pension cost (income) presented in “Note 10 – Employee Benefits.”

[2] Included in interest income on the consolidated statements of income.

[3] Listed as net gain (loss) on sale of securities AFS on the consolidated statements of income.

[4] Included in interest expense for FHLB borrowings on the consolidated statements of income.