ACCUMULATED OTHER COMPREHENSIVE LOSS - Net Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Mar. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortized cost of debt securities transferred to held-to-maturity	$ 305,600		$ 305,600		$ 305,600
After tax unrealized loss of debt securities transferred to held-to-maturity					$ 12,100
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	1,477,169	$ 1,562,280	1,519,679	$ 1,469,955
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			(136,180)
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			1,649
Total other comprehensive loss	(45,617)	(5,960)	(134,531)	(19,831)
Balance at end of period	1,451,862	1,546,060	1,451,862	1,546,060
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(97,453)	4,834	(8,539)	18,705
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax				(20,218)
Reclassifications from accumulated other comprehensive loss to earnings, net of tax				387
Total other comprehensive loss	(45,617)	(5,960)	(134,531)	(19,831)
Balance at end of period	(143,070)	(1,126)	(143,070)	(1,126)
Unrealized Gains/(Losses) on Debt Securities Available-for-Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(336)	28,175
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			(124,097)	(20,218)
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			(36)	(133)
Total other comprehensive loss			(124,133)	(20,351)
Balance at end of period	(124,469)	7,824	(124,469)	7,824
Defined Benefit Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(8,203)	(9,470)
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			0	0
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			419	520
Total other comprehensive loss			419	520
Balance at end of period	(7,784)	$ (8,950)	(7,784)	$ (8,950)
Unrealized Losses on Debt Securities Transferred from Available-for-Sale to Held-to-Maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			0
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			(12,083)
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			1,266
Total other comprehensive loss			(10,817)
Balance at end of period	$ (10,817)		$ (10,817)