Mortgage-Backed Debt - Additional Information (Detail) $ in Billions	12 Months Ended
	Dec. 31, 2016 USD ($)
Beneficial Interests And Related Collateral [Line Items]
Range Of Final Maturities For Residual Trusts	2036 to 2040
Mandatory cleanup calls required when loan pool falls to percent of original principal balance	10.00%
Mortgage-Backed Debt [Member] | Residential Mortgages, Receivables, Real Estate Owned and Restricted Cash [Member]
Beneficial Interests And Related Collateral [Line Items]
Collateral pledged	$ 1.0