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	Mar. 31, 2015	Dec. 31, 2014
Federal National Mortgage Association Certificates And Obligations FNMA Member
Investment Holdings [Line Items]
Amortized Cost	$ 2,008,358	$ 1,746,807
Fair Value	2,015,810	1,736,987
Federal Home Loan Bank Certificates And Obligations FHLB Member
Investment Holdings [Line Items]
Amortized Cost	538,493	737,149
Fair Value	538,874	732,894
Federal Home Loan Mortgage Corporation Certificates And Obligations FHLMC Member
Investment Holdings [Line Items]
Amortized Cost	1,161,089	1,117,865
Fair Value	$ 1,163,815	$ 1,112,485