ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications			$ 23,460	$ 20,698
Amounts reclassified from accumulated other comprehensive income	$ (535)	$ (583)	(5,215)	(3,602)
Other comprehensive income (loss)	4,946	(2,858)	18,245	17,096
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 before reclassifications			23,460	20,698
Amounts reclassified from accumulated other comprehensive income			(4,399)	(2,995)
Other comprehensive income (loss)			19,061	17,703
Balance	68,809	78,072	68,809	78,072
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 before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income			719	1,271
Other comprehensive income (loss)			719	1,271
Balance	(15,580)	(16,035)	(15,580)	(16,035)
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 before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income			(1,535)	(1,878)
Other comprehensive income (loss)			(1,535)	(1,878)
Balance	11,097	13,492	11,097	13,492
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance			(3,667)	(1,936)
Other comprehensive income before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income	(271)	(117)	(816)	(607)
Other comprehensive income (loss)			(816)	(607)
Balance	(4,483)	(2,543)	(4,483)	(2,543)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance			46,081	58,433
Balance	$ 64,326	$ 75,529	$ 64,326	$ 75,529