DEBT AND OTHER OBLIGATIONS - Amount of Collateral Pledged for Repurchase Agreements, Grouped by Asset Class (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	$ 429,819	$ 861,732
Collateral pledged for securities lending transactions	25,621
Total securities	455,440
U.S. Treasury and agency securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	403,199	381,463
Mortgage Loans
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	26,620	480,269
Corporate securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	14,886
Equity securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	10,209
Preferred Stock
Debt Instrument [Line Items]
Collateral pledged for securities lending transactions	526
States and municipal securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Other asset-backed securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Non-U.S. sovereign debt
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Overnight and Continuous
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	363,479	837,974
Collateral pledged for securities lending transactions	25,621
Total securities	389,100
Overnight and Continuous | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	336,859	357,705
Overnight and Continuous | Mortgage Loans
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	26,620	480,269
Overnight and Continuous | Corporate securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	14,886
Overnight and Continuous | Equity securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	10,209
Overnight and Continuous | Preferred Stock
Debt Instrument [Line Items]
Collateral pledged for securities lending transactions	526
Overnight and Continuous | States and municipal securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Overnight and Continuous | Other asset-backed securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Overnight and Continuous | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Up to 30 days
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	66,340	23,758
Collateral pledged for securities lending transactions	0
Total securities	66,340
Up to 30 days | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	66,340	23,758
Up to 30 days | Mortgage Loans
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
Up to 30 days | Corporate securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	0
Up to 30 days | Equity securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	0
Up to 30 days | Preferred Stock
Debt Instrument [Line Items]
Collateral pledged for securities lending transactions	0
Up to 30 days | States and municipal securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Up to 30 days | Other asset-backed securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Up to 30 days | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
30-90 days
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
Collateral pledged for securities lending transactions	0
Total securities	0
30-90 days | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
30-90 days | Mortgage Loans
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
30-90 days | Corporate securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	0
30-90 days | Equity securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	0
30-90 days | Preferred Stock
Debt Instrument [Line Items]
Collateral pledged for securities lending transactions	0
30-90 days | States and municipal securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
30-90 days | Other asset-backed securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
30-90 days | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Greater Than 90 days
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
Collateral pledged for securities lending transactions	0
Total securities	0
Greater Than 90 days | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
Greater Than 90 days | Mortgage Loans
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements	0	0
Greater Than 90 days | Corporate securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	0
Greater Than 90 days | Equity securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Collateral pledged for securities lending transactions	0
Greater Than 90 days | Preferred Stock
Debt Instrument [Line Items]
Collateral pledged for securities lending transactions	$ 0
Greater Than 90 days | States and municipal securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Greater Than 90 days | Other asset-backed securities
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		0
Greater Than 90 days | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Collateral pledged for repurchase agreements		$ 0