HMBS Related Obligations - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Other Liabilities Disclosure [Abstract]
HMBS related obligations at fair value	$ 9,951,895,000	$ 8,652,746,000
Weighted Average Interest Rate On Hmbs Related Obligations	4.18%	4.40%
Average Remaining Life Of Hmbs Related Obligations	3 years 11 months
Residential Loans And Real Estate Owned Pledged As Collateral	9,900,000,000
Loans Pledged As Collateral, Unpaid Principal Balance	$ 9,200,000,000