RESIDENTIAL INVESTMENT SECURITIES - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Residential Investment securities sold, carrying value	$ 2,900.0	$ 2,500.0	$ 3,400.0	$ 4,600.0
Residential Investment securities sold, net realized gain (loss)	$ (63.1)	$ (5.2)	$ (50.0)	$ (4.0)