Residential Investment Securities - Unrealized Loss Position (Detail) $ in Thousands		Mar. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities
Unrealized Loss Position For:
Estimated Fair Value	[1]	$ 23,366,016	$ 41,777,836
Gross Unrealized Losses	[1]	$ (210,679)	$ (854,240)
Number of Securities | Securities	[1]	255	652
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	[1]	$ 2,633,437	$ 20,072,072
Gross Unrealized Losses	[1]	$ (7,176)	$ (164,259)
Number of Securities | Securities	[1]	67	463
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	[1]	$ 20,732,579	$ 21,705,764
Gross Unrealized Losses	[1]	$ (203,503)	$ (689,981)
Number of Securities | Securities	[1]	188	189

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