Name of issuers, and the aggregate amortized cost and fair value of the securities of such issuer, in which the aggregate amortized cost exceeds 10% of stockholders' equity (Detail) (Securities That Exceed 10 Percent Of Stockholders Equity [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Federal National Mortgage Association Certificates And Obligations FNMA Member
Investment Holdings [Line Items]
Amortized Cost	$ 2,147,390	$ 1,594,933
Marketable Securities	2,133,556	1,634,927
Federal Home Loan Bank Certificates And Obligations FHLB Member
Investment Holdings [Line Items]
Amortized Cost	339,886	520,127
Marketable Securities	330,477	528,287
Federal Home Loan Mortgage Corporation Certificates And Obligations FHLMC Member
Investment Holdings [Line Items]
Amortized Cost	1,235,448	1,198,969
Marketable Securities	$ 1,233,785	$ 1,221,863