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, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Initial collateral (as a percent)	102.00%
Fair value of collateral	$ 39,600
Obligation to return securities	39,600
Repurchase agreements and repurchase-to-maturity transactions	856,634	$ 861,732
Securities lending transactions	37,900
Total securities	894,534
Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	842,968	837,974
Securities lending transactions	37,900
Total securities	880,868
Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	13,666	23,758
Securities lending transactions	0
Total securities	13,666
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	360,106	381,463
U.S. Treasury and agency securities | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	346,440	357,705
U.S. Treasury and agency securities | Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	13,666	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
Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	496,528	480,269
Mortgage loans | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	496,528	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	35,557
Corporate Securities | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	35,557
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	1,907
Equity securities | Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	1,907
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	436
Preferred stock | Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	436
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
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
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