Schedule of Agency Securities in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	$ 15,637,055	$ 191,580
Unrealized Loss - Less than 12 months	(343,090)	(121)
Fannie Mae Certificates, ARMs
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	7,100,520	149,954
Unrealized Loss - Less than 12 months	(147,577)	(100)
Fannie Mae Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	2,067,011
Unrealized Loss - Less than 12 months	(60,120)
Freddie Mac Certificates, ARMs
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	5,775,068	41,625
Unrealized Loss - Less than 12 months	(115,264)	(21)
Freddie Mac Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	694,456
Unrealized Loss - Less than 12 months	$ (20,129)