Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013 Securities	Dec. 31, 2012 Securities
Unrealized Loss Position For:
Estimated Fair Value	$ 53,779,480	$ 11,368,289
Gross Unrealized Losses	(3,053,077)	(188,001)
Number of Securities	638	226
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	47,677,197	11,220,514
Gross Unrealized Losses	(2,569,474)	(82,721)
Number of Securities	583	187
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	6,102,283	147,775
Gross Unrealized Losses	$ (483,603)	$ (105,280)
Number of Securities	55	39