ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
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	$ 20,568	(18,969)
Amounts reclassified from accumulated other comprehensive income (loss)	(98)	(348)
Other comprehensive income (loss)	20,470	(19,317)
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)	20,470	(19,317)
Unrealized gains (losses) on available-for- sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	7,724	83,497
Cumulative adjustment for adoption of financial instruments recognition and measurement changes		(66,234)
Other comprehensive income (loss) before reclassifications	20,568	(18,969)
Amounts reclassified from accumulated other comprehensive income (loss)	215	193
Other comprehensive income (loss)	20,783	(18,776)
Balance	28,507	(1,513)
Unrecognized pension and postretirement benefit obligations, net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(17,626)	(13,074)
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)	197	81
Other comprehensive income (loss)	197	81
Balance	(17,429)	(12,993)
Unrecognized pension and postretirement benefit obligations, prior service credit
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	11,522	12,961
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)	(510)	(622)
Other comprehensive income (loss)	(510)	(622)
Balance	11,012	12,339
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(6,104)	(113)
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)	(313)	(541)
Other comprehensive income (loss)	(313)	(541)
Balance	(6,417)	(654)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	1,620	83,384
Cumulative adjustment for adoption of financial instruments recognition and measurement changes		(66,234)
Balance	$ 22,090	$ (2,167)

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 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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