Schedule of Agency Securities in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013 Securities	Dec. 31, 2012 Securities
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	$ 10,239,516	$ 191,580
Unrealized Loss - Less than 12 months	(183,020)	(121)
Number of securities in an unrealized loss position	318	4
Fannie Mae Certificates, ARMs
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	4,939,333	149,954
Unrealized Loss - Less than 12 months	(84,364)	(100)
Fannie Mae Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	322,691
Unrealized Loss - Less than 12 months	(10,880)
Freddie Mac Certificates, ARMs
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	4,977,492	41,625
Unrealized Loss - Less than 12 months	$ (87,776)	$ (21)