DEBT AND OTHER OBLIGATIONS - Amount of Collateral Pledged for Repurchase Agreements, Grouped by Asset Class (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	$ 549,788	$ 861,732
Securities lending transactions	100,180
Total securities	649,968
U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	342,992	381,463
Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	206,796	480,269
Corporate securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	97,385
Equity securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	2,621
Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	174
State and municipal securities
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
Non-U.S. sovereign debt
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	529,782	837,974
Securities lending transactions	100,180
Total securities	629,962
Overnight and Continuous | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	322,986	357,705
Overnight and Continuous | Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	206,796	480,269
Overnight and Continuous | Corporate securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	97,385
Overnight and Continuous | Equity securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	2,621
Overnight and Continuous | Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	174
Overnight and Continuous | State and municipal securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Overnight and Continuous | Other asset-backed securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Overnight and Continuous | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	20,006	23,758
Securities lending transactions	0
Total securities	20,006
Up to 30 days | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	20,006	23,758
Up to 30 days | Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Up to 30 days | Corporate securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Up to 30 days | Equity securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Up to 30 days | Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	0
Up to 30 days | State and municipal securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Up to 30 days | Other asset-backed securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Up to 30 days | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Securities lending transactions	0
Total securities	0
30-90 days | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
30-90 days | Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
30-90 days | Corporate securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
30-90 days | Equity securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
30-90 days | Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	0
30-90 days | State and municipal securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
30-90 days | Other asset-backed securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
30-90 days | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		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
Greater Than 90 days | U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Greater Than 90 days | Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Greater Than 90 days | Corporate securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Greater Than 90 days | Equity securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Securities lending transactions	0
Greater Than 90 days | Preferred stock
Debt Instrument [Line Items]
Securities lending transactions	$ 0
Greater Than 90 days | State and municipal securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Greater Than 90 days | Other asset-backed securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Greater Than 90 days | Non-U.S. sovereign debt
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		$ 0