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, USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Mortgage-backed securities - FNMA
Investment Holdings [Line Items]
Amortized Cost	$ 1,008,700	$ 757,812	$ 963,714
Fair Value	1,045,160	789,838	996,966
FHLB
Investment Holdings [Line Items]
Amortized Cost	813,337	1,003,395	1,418,562
Fair Value	855,844	1,056,549	1,486,376
Freddie Mac
Investment Holdings [Line Items]
Amortized Cost	959,192	637,644	624,844
Fair Value	$ 983,969	$ 654,495	$ 638,388