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	Jun. 30, 2015	Dec. 31, 2014
Federal National Mortgage Association Certificates And Obligations FNMA Member
Investment Holdings [Line Items]
Amortized Cost	$ 2,274,845	$ 1,746,807
Fair Value	2,264,445	1,736,987
Federal Home Loan Bank Certificates And Obligations FHLB Member
Investment Holdings [Line Items]
Amortized Cost	445,169	737,149
Fair Value	444,409	732,894
Federal Home Loan Mortgage Corporation Certificates And Obligations FHLMC Member
Investment Holdings [Line Items]
Amortized Cost	962,147	1,117,865
Fair Value	$ 959,054	$ 1,112,485