DEBT AND OTHER OBLIGATIONS - Amount of Collateral Pledged for Repurchase Agreements and Securities Lending Transactions, Grouped by Asset Class (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Initial collateral (as a percent)	102.00%
Securities lending transactions	$ 92,022	$ 125,271
Fair value of collateral	97,300
Obligation to return securities	97,300
Repurchase agreements and repurchase-to-maturity transactions	457,524	1,006,607
Total securities	549,546	1,131,878
Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	92,022	125,271
Repurchase agreements and repurchase-to-maturity transactions	401,011	1,006,607
Total securities	493,033	1,131,878
Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	56,513	0
Total securities	56,513	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	457,524	307,633
U.S. Treasury and agency securities \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	401,011	307,633
U.S. Treasury and agency securities \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	56,513	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
Other asset-backed securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Other asset-backed securities \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Other asset-backed securities \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Other asset-backed securities \| 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0
Other 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	0	698,974
Mortgage loans \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	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
Fixed maturities
Debt Instrument [Line Items]
Securities lending transactions	91,040
Fixed maturities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	91,040
Fixed maturities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0
Fixed maturities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Fixed maturities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Corporate securities
Debt Instrument [Line Items]
Securities lending transactions		118,817
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions		118,817
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions		0
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions		0
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions		0
Repurchase agreements and repurchase-to-maturity transactions		0
Equity securities
Debt Instrument [Line Items]
Securities lending transactions	982	5,699
Equity securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	982	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
Redeemable preferred stock
Debt Instrument [Line Items]
Securities lending transactions	0	755
Redeemable preferred stock \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	0	755
Redeemable preferred stock \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Redeemable preferred stock \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Redeemable preferred stock \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0	$ 0
Other government related securities
Debt Instrument [Line Items]
Securities lending transactions	0
Other government related securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	0
Other government related securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0
Other government related securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Other government related securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	$ 0

Secured Borrowings, Percent Of Initial Collateral Required

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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.

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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.

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Fair value of the securities received as collateral against securities borrowed.

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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.

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