ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Reclassifications out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Amounts Reclassified from AOCI
Interest expense	$ 9,639	$ 8,375	$ 30,666	$ 22,822
Income tax (expense) benefit	(3,852)	(1,381)	(10,126)	(6,263)
Net income	13,903	6,547	37,483	25,363
Amount Reclassified from Accumulated Other Comprehensive Income
Amounts Reclassified from AOCI
Total reclassifications, before income tax	203	280	1,169	(7,552)
Income tax (expense) benefit	(59)	(81)	(341)	2,197
Net income	144	199	828	(5,355)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized (losses) gains on sale of available for sale securities | Net securities gains (losses)
Amounts Reclassified from AOCI
Net securities gains (losses)			201	(7,921)
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service credit | Other operating expense
Amounts Reclassified from AOCI
Other operating expenses	20	20	58	58
Amount Reclassified from Accumulated Other Comprehensive Income | Transition obligation | Other operating expense
Amounts Reclassified from AOCI
Other operating expenses		(1)		(3)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses | Other operating expense
Amounts Reclassified from AOCI
Other operating expenses	(148)	(113)	(443)	(339)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized gains on cash flow hedges | Interest expense
Amounts Reclassified from AOCI
Interest expense	$ 331	$ 374	$ 1,353	$ 653