Schedule of Agency Securities in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	$ 578,212	$ 191,580
Unrealized Loss - Less than 12 months	(1,568)	(121)
Fannie Mae Certificates, ARMs
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	148,159	149,954
Unrealized Loss - Less than 12 months	(83)	(100)
Fannie Mae Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	323,387
Unrealized Loss - Less than 12 months	(1,457)
Freddie Mac Certificates, ARMs
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	51,353	41,625
Unrealized Loss - Less than 12 months	(20)	(21)
Freddie Mac Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	55,313
Unrealized Loss - Less than 12 months	$ (8)