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, 2012	Dec. 31, 2011	Mar. 31, 2011
Federal National Mortgage Association Certificates And Obligations FNMA Member
Investment Holdings [Line Items]
Amortized Cost	$ 1,117,928	$ 1,049,315	$ 1,029,936
Marketable Securities	1,157,729	1,089,069	1,057,977
Federal Home Loan Bank Certificates And Obligations FHLB Member
Investment Holdings [Line Items]
Amortized Cost	583,897	553,940	1,003,317
Marketable Securities	604,020	578,617	1,047,747
Federal Home Loan Mortgage Corporation Certificates And Obligations FHLMC Member
Investment Holdings [Line Items]
Amortized Cost	1,149,415	984,270	977,365
Marketable Securities	$ 1,176,487	$ 1,010,669	$ 993,342