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	Sep. 30, 2014	Dec. 31, 2013
Federal National Mortgage Association Certificates And Obligations FNMA Member
Investment Holdings [Line Items]
Amortized Cost	$ 1,883,634	$ 2,318,171
Marketable Securities	1,853,487	2,266,610
Federal Home Loan Bank Certificates And Obligations FHLB Member
Investment Holdings [Line Items]
Amortized Cost	887,135	336,933
Marketable Securities	880,513	326,220
Federal Home Loan Mortgage Corporation Certificates And Obligations FHLMC Member
Investment Holdings [Line Items]
Amortized Cost	1,292,586	1,434,346
Marketable Securities	$ 1,282,920	$ 1,418,216