SECURITIES - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Residential Investment securities sold, carrying value	$ 9,100.0	$ 6,800.0	$ 14,500.0	$ 11,400.0
Residential Investment securities sold, net realized gain (loss)	$ (322.4)	$ (10.2)	$ (372.5)	$ (14.3)