Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Mortgage-Backed Securities in a continuous unrealized loss position [Line Items]
Unrealized Loss Position For Less than 12 Months - Estimated Fair Value	$ 20,911,053	$ 28,608,996
Unrealized Loss Position For Less than 12 Months - Unrealized Losses	(302,761)	(577,096)
Unrealized Loss Position For 12 Months or More - Estimated Fair Value	172,421	166,481
Unrealized Loss Position For 12 Months or More - Unrealized Losses	(5,108)	(3,929)
Total Unrealized Loss Position - Estimated Fair Value	21,083,474	28,775,477
Total Unrealized Loss Position - Unrealized Losses	$ (307,869)	$ (581,025)