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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amounts Reclassified from AOCI
Interest expense	$ 9,382	$ 8,375	$ 7,622	$ 6,825	$ 6,399	$ 6,093	$ 5,441	$ 4,756	$ 32,204	$ 22,689	$ 16,845
Income tax benefit	2,878	1,381	1,701	3,181	5,422	4,703	4,505	4,316	9,141	18,946	18,795
Total reclassifications, net of income tax	$ 13,864	$ 6,547	$ 6,743	$ 12,073	$ (6,916)	$ 9,443	$ 8,841	$ 9,171	39,227	20,539	35,491
Amount Reclassified from Accumulated Other Comprehensive Income
Amounts Reclassified from AOCI
Total reclassifications, before income tax									(7,237)	(1,861)	(879)
Income tax benefit									(2,105)	(762)	(356)
Total reclassifications, net of income tax									(5,132)	(1,099)	(523)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized (losses) gains on sale of available for sale securities
Amounts Reclassified from AOCI
Net securities gains (losses)									(7,921)	38	449
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service credit
Amounts Reclassified from AOCI
Other operating expenses									77	77	77
Amount Reclassified from Accumulated Other Comprehensive Income | Transition obligation
Amounts Reclassified from AOCI
Other operating expenses									(5)	(27)	(28)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses
Amounts Reclassified from AOCI
Other operating expenses									(456)	(530)	(433)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized gains (losses) on cash flow hedges
Amounts Reclassified from AOCI
Interest expense									$ 1,068	$ (1,419)	$ (944)