Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014 Securities	Dec. 31, 2013 Securities
Unrealized Loss Position For:
Estimated Fair Value	$ 56,135,070	$ 53,779,480
Gross Unrealized Losses	(2,450,905)	(3,053,077)
Number of Securities	692	638
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	50,079,785	47,677,197
Gross Unrealized Losses	(2,046,126)	(2,569,474)
Number of Securities	642	583
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	6,055,285	6,102,283
Gross Unrealized Losses	$ (404,779)	$ (483,603)
Number of Securities	50	55