ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	$ 0
Other comprehensive income (loss) before reclassifications	(18,969)	$ 9,705
Amounts reclassified from accumulated other comprehensive income (loss)	(348)	(1,351)
Other comprehensive income (loss)	(19,317)	8,354
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	(66,234)
Other comprehensive income (loss)	(19,317)	8,354
Unrealized gains (losses) on available-for- sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	83,497	49,748
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	(66,234)
Other comprehensive income (loss) before reclassifications	(18,969)	9,705
Amounts reclassified from accumulated other comprehensive income (loss)	193	(1,079)
Other comprehensive income (loss)	(18,776)	8,626
Balance	(1,513)	58,374
Unrecognized pension and postretirement benefit obligations, net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(13,074)	(16,299)
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	0
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	81	239
Other comprehensive income (loss)	81	239
Balance	(12,993)	(16,060)
Unrecognized pension and postretirement benefit obligations, prior service credit
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	12,961	12,632
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	0
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(622)	(511)
Other comprehensive income (loss)	(622)	(511)
Balance	12,339	12,121
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(113)	(3,667)
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	0
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(541)	(272)
Other comprehensive income (loss)	(541)	(272)
Balance	(654)	(3,939)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	83,384	46,081
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	(66,234)
Balance	$ (2,167)	$ 54,435