ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	$ 0		$ 0
Other comprehensive income (loss) before reclassifications			(26,546)	$ 17,979
Amounts reclassified from accumulated other comprehensive income (loss)	3,800	$ (3,329)	3,452	(4,680)
Other comprehensive income (loss)	(3,777)	4,945	(23,094)	13,299
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)		(66,234)
Other comprehensive income (loss)			(23,094)	13,299
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)		(66,234)
Other comprehensive income (loss) before reclassifications			(26,546)	17,979
Amounts reclassified from accumulated other comprehensive income (loss)			4,530	(4,135)
Other comprehensive income (loss)			(22,016)	13,844
Balance	(4,753)	63,592	(4,753)	63,592
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		0
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)			166	479
Other comprehensive income (loss)			166	479
Balance	(12,908)	(15,820)	(12,908)	(15,820)
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		0
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)			(1,244)	(1,024)
Other comprehensive income (loss)			(1,244)	(1,024)
Balance	11,717	11,608	11,717	11,608
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		0
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(537)	(273)	(1,078)	(545)
Other comprehensive income (loss)			(1,078)	(545)
Balance	(1,191)	(4,212)	(1,191)	(4,212)
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)		(66,234)
Balance	$ (5,944)	$ 59,380	$ (5,944)	$ 59,380

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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