Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 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	$ 1,087,552	$ 28,608,996
Unrealized Loss Position For Less than 12 Months - Unrealized Losses	(118,593)	(577,096)
Unrealized Loss Position For 12 Months or More - Estimated Fair Value	883,143	166,481
Unrealized Loss Position For 12 Months or More - Unrealized Losses	(16,529)	(3,929)
Total Unrealized Loss Position - Estimated Fair Value	1,970,695	28,775,477
Total Unrealized Loss Position - Unrealized Losses	$ (135,122)	$ (581,025)