Schedule of Agency Securities in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	$ 233,755
Unrealized Loss - Less than 12 months	(138)
Fannie Mae Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	210,743
Unrealized Loss - Less than 12 months	(132)
Freddie Mac Certificates, Fixed Rate
Investments, Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	23,012
Unrealized Loss - Less than 12 months	$ (6)