Residential Investment Securities - Unrealized Loss Position (Detail) $ in Thousands		Sep. 30, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities
Unrealized Loss Position For:
Estimated Fair Value	[1]	$ 15,072,658	$ 41,777,836
Gross Unrealized Losses	[1]	$ (60,599)	$ (854,240)
Number of Securities | Securities	[1]	499	652
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	[1]	$ 8,204,957	$ 20,072,072
Gross Unrealized Losses	[1]	$ (28,676)	$ (164,259)
Number of Securities | Securities	[1]	445	463
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	[1]	$ 6,867,701	$ 21,705,764
Gross Unrealized Losses	[1]	$ (31,923)	$ (689,981)
Number of Securities | Securities	[1]	54	189

[1] Excludes interest-only mortgage-backed securities.