DEBT AND OTHER OBLIGATIONS - Amount of Collateral Pledged for Repurchase Agreements and Securities Lending Transactions, Grouped by Asset Class (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Initial collateral (as a percent)	102.00%
Fair value of collateral	$ 26,500
Obligation to return securities	26,500
Repurchase agreements and repurchase-to-maturity transactions	429,819	$ 861,732
Securities lending transactions	25,621
Total securities	455,440
Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	363,479	837,974
Securities lending transactions	25,621
Total securities	389,100
Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	66,340	23,758
Securities lending transactions	0
Total securities	66,340
30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Securities lending transactions	0
Total securities	0
Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Securities lending transactions	0
Total securities	0
U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	403,199	381,463
U.S. Treasury and agency securities | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	336,859	357,705
U.S. Treasury and agency securities | Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	66,340	23,758
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
States, municipals, and political subdivisions
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
States, municipals, and political subdivisions | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
States, municipals, and political subdivisions | Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
States, municipals, and political subdivisions | 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
States, municipals, and political subdivisions | Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		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	26,620	480,269
Mortgage loans | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	26,620	480,269
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]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	14,886
Corporate Securities | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	14,886
Corporate Securities | Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Corporate Securities | 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Corporate Securities | Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Equity securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	10,209
Equity securities | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	10,209
Equity securities | Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Equity securities | 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Equity securities | Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	526
Preferred stock | Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	526
Preferred stock | Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0
Preferred stock | 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Preferred stock | Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	$ 0
Non-U.S. sovereign debt
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Non-U.S. sovereign debt | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Non-U.S. sovereign debt | Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Non-U.S. sovereign debt | 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Non-U.S. sovereign debt | Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		$ 0