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, 2012	Dec. 31, 2011
Federal National Mortgage Association Certificates And Obligations FNMA Member
Investment Holdings [Line Items]
Amortized Cost	$ 1,234,776	$ 1,049,315
Marketable Securities	1,273,616	1,089,069
Federal Home Loan Bank Certificates And Obligations FHLB Member
Investment Holdings [Line Items]
Amortized Cost	538,854	553,940
Marketable Securities	555,078	578,617
Federal Home Loan Mortgage Corporation Certificates And Obligations FHLMC Member
Investment Holdings [Line Items]
Amortized Cost	1,123,166	984,270
Marketable Securities	$ 1,147,089	$ 1,010,669