DEBT AND OTHER OBLIGATIONS - Secured Financing Transactions (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Initial collateral required (as a percent)	102.00%
Securities lending transactions	$ 100,180
Fair value of collateral	105,800
Obligation to return collateral	105,800
Repurchase Program Borrowings
Debt Instrument [Line Items]
Fair value of securities pledged under the repurchase program	549,800	$ 861,700
Secured financing liabilities	$ 493,800	$ 797,700
Average borrowing rate (as a percent)	1.18%	0.65%
Maximum outstanding balance	$ 981,300	$ 1,065,800
Average daily balance	$ 546,200	$ 505,400
Average borrowing rate (as a percent)	0.89%	0.44%
Repurchase Program Borrowings | Maximum
Debt Instrument [Line Items]
Term of financing agreement	90 days