Residential Investment Securities - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Residential Investment securities sold, carrying value	$ 3,800.0	$ 3,700.0	$ 9,100.0	$ 21,100.0
Residential Investment securities sold, realized gain	$ 14.7	$ 4.5	$ 14.7	$ 70.8