Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014 Securities	Dec. 31, 2013 Securities
Unrealized Loss Position For:
Estimated Fair Value	$ 49,605,740	$ 53,779,480
Gross Unrealized Losses	(1,597,019)	(3,053,077)
Number of Securities	565	638
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	10,279,297	47,677,197
Gross Unrealized Losses	(55,136)	(2,569,474)
Number of Securities	202	583
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	39,326,443	6,102,283
Gross Unrealized Losses	$ (1,541,883)	$ (483,603)
Number of Securities	363	55