ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
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			$ 38,820	(26,546)
Amounts reclassified from accumulated other comprehensive income (loss)	$ (366)	3,800	(464)	3,452
Other comprehensive income (loss)	17,886	(3,777)	38,356	(23,094)
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)	17,886	(3,777)	38,356	(23,094)
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)		(66,234)
Other comprehensive income (loss) before reclassifications			38,820	(26,546)
Amounts reclassified from accumulated other comprehensive income (loss)			175	4,530
Other comprehensive income (loss)			38,995	(22,016)
Balance	46,719	(4,753)	46,719	(4,753)
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		0
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)			382	166
Other comprehensive income (loss)			382	166
Balance	(17,244)	(12,908)	(17,244)	(12,908)
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		0
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)			(1,021)	(1,244)
Other comprehensive income (loss)			(1,021)	(1,244)
Balance	10,501	11,717	10,501	11,717
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		0
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(326)	(537)	(639)	(1,078)
Other comprehensive income (loss)			(639)	(1,078)
Balance	(6,743)	(1,191)	(6,743)	(1,191)
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)		(66,234)
Balance	$ 39,976	$ (5,944)	$ 39,976	$ (5,944)

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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