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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassifications out of accumulated other comprehensive (loss) income
Interest expense	$ 4,492	$ 5,589	$ 5,418	$ 7,952	$ 8,672	$ 9,639	$ 10,835	$ 10,192	$ 23,451	$ 39,338	$ 32,204
Loss on termination of swaps									(3,403)
Income tax benefit (expense)	(3,881)	(3,999)	(3,129)	(2,676)	(3,934)	(3,852)	(2,859)	(3,415)	(13,685)	(14,060)	(9,141)
Net income	$ 8,977	$ 13,061	$ 10,656	$ 9,348	$ 14,208	$ 13,903	$ 10,653	$ 12,927	42,042	51,691	39,227
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income
Reclassifications out of accumulated other comprehensive (loss) income
Total reclassifications, before income tax									(2,072)	1,302	(7,237)
Net income									(1,466)	922	(5,132)
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Loss on termination of swaps
Reclassifications out of accumulated other comprehensive (loss) income
Loss on termination of swaps									(3,365)
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Income tax expense
Reclassifications out of accumulated other comprehensive (loss) income
Income tax benefit (expense)									606	(380)	2,105
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Realized gains (losses) on sale of available for sale securities | Net securities gains (losses)
Reclassifications out of accumulated other comprehensive (loss) income
Net securities gains									3,525	201	(7,921)
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Prior service credit | Other operating expenses
Reclassifications out of accumulated other comprehensive (loss) income
Other operating expenses									77	77	77
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Transition obligation | Other operating expenses
Reclassifications out of accumulated other comprehensive (loss) income
Other operating expenses											(5)
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Actuarial losses | Other operating expenses
Reclassifications out of accumulated other comprehensive (loss) income
Other operating expenses									(658)	(564)	(456)
Amount Reclassified from Accumulated Other Comprehensiven (loss) Income | Realized (losses) gains on cash flow hedges | Interest expense.
Reclassifications out of accumulated other comprehensive (loss) income
Interest expense									$ (1,651)	$ 1,588	$ 1,068