CAPITAL RATIOS AND SHAREHOLDERS' EQUITY - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,164,140	$ 1,779,837	$ 1,758,564	$ 1,552,918
Other comprehensive income (loss) before reclassifications	(809)	4,583	20,437	29,357
Less: amounts reclassified from accumulated other comprehensive income	4	4	4	7
Total other comprehensive income	(813)	4,579	20,433	29,350
Balance at end of period	1,179,025	1,772,224	1,179,025	1,772,224
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period	33,239	11,301	11,993	(13,470)
Total other comprehensive income	(813)	4,579	20,433	29,350
Balance at end of period	32,426	15,880	32,426	15,880
Net unrealized holding gain on AFS securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period	35,450	13,318	14,204	(11,453)
Other comprehensive income (loss) before reclassifications	(809)	4,583	20,437	29,357
Less: amounts reclassified from accumulated other comprehensive income	4	4	4	7
Total other comprehensive income	(813)	4,579	20,433	29,350
Balance at end of period	34,637	17,897	34,637	17,897
Net unrealized holding (loss) on pension plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period	(2,211)	(2,017)	(2,211)	(2,017)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Less: amounts reclassified from accumulated other comprehensive income	0	0	0	0
Total other comprehensive income	0	0	0	0
Balance at end of period	$ (2,211)	$ (2,017)	$ (2,211)	$ (2,017)