DEBT AND OTHER OBLIGATIONS - Repurchase Program Borrowings (Details) - Repurchase Program Borrowings - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Fair value of securities pledged under the repurchase program	$ 549.8	$ 861.7
Secured financing liabilities	493.8	797.7
Maximum balance outstanding	$ 981.3	$ 1,065.8
Average daily balance, average borrowing rate (as a percent)	0.89%	0.44%
Average borrowing rate (as a percent)	1.18%	0.65%
Average daily balance	$ 546.2	$ 505.4
Maximum
Debt Instrument [Line Items]
Term of financing agreement	90 days