ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications	$ 32,945	$ (4,766)
Amounts reclassified from accumulated other comprehensive income	(9,415)	(7,586)	$ (6,243)
Other comprehensive income (loss)	23,530	(12,352)	(23,229)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	0
Unrealized gains (losses) on available-for- sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	49,748	60,369
Other comprehensive income (loss) before reclassifications	27,278	(3,885)
Amounts reclassified from accumulated other comprehensive income	(8,326)	(6,736)
Other comprehensive income (loss)	18,952	(10,621)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	14,797
Balance	83,497	49,748	60,369
Unrecognized pension and postretirement benefit obligations, net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(16,299)	(17,306)
Other comprehensive income (loss) before reclassifications	5,571	(542)
Amounts reclassified from accumulated other comprehensive income	958	1,549
Other comprehensive income (loss)	6,529	1,007
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	(3,304)
Balance	(13,074)	(16,299)	(17,306)
Unrecognized pension and postretirement benefit obligations, prior service credit
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	12,632	15,370
Other comprehensive income (loss) before reclassifications	96	(339)
Amounts reclassified from accumulated other comprehensive income	(2,047)	(2,399)
Other comprehensive income (loss)	(1,951)	(2,738)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	2,280
Balance	12,961	12,632	15,370
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(3,667)	(1,936)
Other comprehensive income (loss) before reclassifications	5,667	(881)
Amounts reclassified from accumulated other comprehensive income	(1,089)	(850)	(1,287)
Other comprehensive income (loss)	4,578	(1,731)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	(1,024)
Balance	(113)	(3,667)	(1,936)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	46,081	58,433
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	13,773
Balance	$ 83,384	$ 46,081	$ 58,433