ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Reclassifications out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Amounts Reclassified from AOCI
Interest expense	$ 10,835	$ 7,622	$ 21,027	$ 14,447
Income tax (expense) benefit	(2,859)	(1,701)	(6,274)	(4,882)
Net income	10,653	6,743	23,580	18,816
Realized (losses) gains on sale of available for sale securities
Amounts Reclassified from AOCI
Net securities gains (losses)	201	(7,921)	201	(7,921)
Amount Reclassified from Accumulated Other Comprehensive Income
Amounts Reclassified from AOCI
Total reclassifications, before income tax	539	(7,672)	966	(7,832)
Income tax (expense) benefit	(157)	2,231	(282)	2,278
Net income	382	(5,441)	684	(5,554)
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service credit
Amounts Reclassified from AOCI
Other operating expenses	19	19	38	38
Amount Reclassified from Accumulated Other Comprehensive Income | Transition obligation
Amounts Reclassified from AOCI
Other operating expenses		(1)		(2)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses
Amounts Reclassified from AOCI
Other operating expenses	(147)	(113)	(295)	(226)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized gains (losses) on cash flow hedges
Amounts Reclassified from AOCI
Interest expense	$ 466	$ 344	$ 1,022	$ 279