DEBT AND OTHER OBLIGATIONS - Amount of Collateral Pledged for Repurchase Agreements and Securities Lending Transactions, Grouped by Asset Class (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Initial collateral (as a percent)	102.00%
Securities lending transactions	$ 107,028	$ 125,271
Fair value of collateral	113,900
Obligation to return securities	113,900
Repurchase agreements and repurchase-to-maturity transactions	753,611	1,006,607
Total securities	860,639	1,131,878
Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	107,028	125,271
Repurchase agreements and repurchase-to-maturity transactions	753,611	1,006,607
Total securities	860,639	1,131,878
Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	0	0
Total securities	0	0
30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	0	0
Total securities	0	0
Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	0	0
Total securities	0	0
U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	293,539	307,633
U.S. Treasury and agency securities \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	293,539	307,633
U.S. Treasury and agency securities \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
U.S. Treasury and agency securities \| 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
U.S. Treasury and agency securities \| Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Asset-backed securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Asset-backed securities \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Asset-backed securities \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Asset-backed securities \| 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Asset-backed securities \| Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	460,072	698,974
Mortgage loans \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	460,072	698,974
Mortgage loans \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Mortgage loans \| 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Mortgage loans \| Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Corporate securities
Debt Instrument [Line Items]
Securities lending transactions	94,155	118,817
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	94,155	118,817
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions		0
Equity securities
Debt Instrument [Line Items]
Securities lending transactions	12,798	5,699
Equity securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	12,798	5,699
Equity securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Equity securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Equity securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	75	755
Preferred stock \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	75	755
Preferred stock \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Preferred stock \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Preferred stock \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	$ 0	$ 0

Secured Borrowings, Percent Of Initial Collateral Required

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+ References

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Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a financial asset repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same financial asset at a date certain for a specified price. Includes repurchase-to-maturity transactions and liabilities not subject to a master netting arrangement.

+ References

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- Definition

Amount, before effects of master netting arrangements, of financial assets loaned to entities in exchange for collateral and funds outstanding borrowed in the form of a financial asset repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same financial assets at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 7
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL34722452-111729

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 7
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL34722452-111729

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Amount of obligation to return securities collateral under master netting arrangements that have not been offset against securities borrowed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226016-175313

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- Definition

Fair value of the securities received as collateral against securities borrowed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226013-175313

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Amount, before effects of master netting arrangements, of financial assets loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and financial assets transferred other than securities that qualify as secured borrowings.

+ References

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