ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amounts Reclassified from AOCI
Interest expense	$ 8,672	$ 9,639	$ 10,835	$ 10,192	$ 9,382	$ 8,375	$ 7,622	$ 6,825	$ 39,338	$ 32,204	$ 22,689
Income tax (expense) benefit	(3,934)	(3,852)	(2,859)	(3,415)	(2,878)	(1,381)	(1,701)	(3,181)	(14,060)	(9,141)	(18,946)
Net income	$ 14,208	$ 13,903	$ 10,653	$ 12,927	$ 13,864	$ 6,547	$ 6,743	$ 12,073	51,691	39,227	20,539
Amount Reclassified from Accumulated Other Comprehensive Income
Amounts Reclassified from AOCI
Total reclassifications, before income tax									1,302	(7,237)	(1,861)
Net income									922	(5,132)	(1,099)
Amount Reclassified from Accumulated Other Comprehensive Income | Interest expense.
Amounts Reclassified from AOCI
Income tax (expense) benefit									(380)	2,105	762
Amount Reclassified from Accumulated Other Comprehensive Income | Realized gains (losses) on sale of available for sale securities | Net securities gains (losses)
Amounts Reclassified from AOCI
Net securities gains (losses)									201	(7,921)	38
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service credit | Other operating expense
Amounts Reclassified from AOCI
Other operating expenses									77	77	77
Amount Reclassified from Accumulated Other Comprehensive Income | Transition obligation | Other operating expense
Amounts Reclassified from AOCI
Other operating expenses										(5)	(27)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses | Other operating expense
Amounts Reclassified from AOCI
Other operating expenses									(564)	(456)	(530)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized gains (losses) on cash flow hedges | Interest expense.
Amounts Reclassified from AOCI
Net securities gains (losses)									$ 1,588	$ 1,068	$ (1,419)