CAPITAL RATIOS AND SHAREHOLDERS' EQUITY - Components of AOCI (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity	$ 1,179,025	$ 1,164,140	$ 1,758,564	$ 1,772,224	$ 1,779,837	$ 1,552,918
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity	32,426	33,239	11,993	15,880	11,301	(13,470)
Net unrealized holding gain on AFS securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other accumulated comprehensive income (loss), before tax	46,792		19,263
Income taxes related to items of accumulated other comprehensive income	(12,155)		(5,059)
Total shareholders’ equity	34,637	35,450	14,204	17,897	13,318	(11,453)
Net unrealized holding (loss) on pension plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other accumulated comprehensive income (loss), before tax	(3,023)		(3,023)
Income taxes related to items of accumulated other comprehensive income	812		812
Total shareholders’ equity	$ (2,211)	$ (2,211)	$ (2,211)	$ (2,017)	$ (2,017)	$ (2,017)