Summary of Significant Accounting Policies - Mortgage Banking and Guaranty Obligations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Gains from Mortgage Banking Activities
Co-broker fees from mortgage banking activities	$ 18.0	$ 15.9	$ 23.0
Fannie Mae DUS program | Minimum
Guaranty Obligation and Allowance for Credit Losses
Average expected cash flows (as a percent)	0.03%
Discount rate (as a percent)	12.00%
Fannie Mae DUS program | Maximum
Guaranty Obligation and Allowance for Credit Losses
Average expected cash flows (as a percent)	0.05%
Discount rate (as a percent)	15.00%
Maximum delinquency period of loans at which initial loss recognition occurs	60 days