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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Amounts Reclassified from AOCI
Interest expense	$ 8,375	$ 6,093	$ 22,822	$ 16,290
Income tax (benefit) expense	1,381	4,703	6,263	13,524
Amount Reclassified from Accumulated Other Comprehensive Income
Amounts Reclassified from AOCI
Total reclassifications, before income tax	280	(221)	(7,552)	(1,182)
Income tax (benefit) expense	(81)	91	2,197	484
Total reclassifications, net of income tax	199	(130)	(5,355)	(698)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized (gains) losses on sale of available for sale securities
Amounts Reclassified from AOCI
Net securities gains (losses)	0	260	(7,921)	260
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service credit
Amounts Reclassified from AOCI
Other operating expenses	20	20	58	58
Amount Reclassified from Accumulated Other Comprehensive Income | Transition obligation
Amounts Reclassified from AOCI
Other operating expenses	(1)	(7)	(3)	(21)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses
Amounts Reclassified from AOCI
Other operating expenses	(113)	(125)	(339)	(376)
Amount Reclassified from Accumulated Other Comprehensive Income | Realized losses on cash flow hedges
Amounts Reclassified from AOCI
Interest expense	$ 374	$ (369)	$ 653	$ (1,103)