Estimated Fair Values of Securities with Gross Unrealized Losses in a Continuous Unrealized Loss Position for Less Than Twelve Months and for Twelve Months or Longer (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Available-for-Sale, Less Than Twelve Months Estimated Fair Value	$ 1,358	$ 2,051
Available-for-Sale,Less Than Twelve Months Gross Unrealized Losses	(12)	(4)
Available-for-Sale, Twelve Months or Longer Estimated Fair Value	17,651	19,991
Available-for-Sale, Twelve Months or Longer Gross Unrealized Losses	(332)	(375)
Available-for-Sale, Total Estimated Fair Value	19,009	22,042
Available-for-Sale, Total Gross Unrealized Losses	(344)	(379)
Held-to-Maturity, Less Than Twelve Months Estimated Fair Value	221,195	508,898
Held-to-Maturity, Less Than Twelve Months Gross Unrealized Losses	(460)	(9,202)
Held-to-Maturity, Twelve Months or Longer Estimated Fair Value		1,744
Held-to-Maturity-Twelve Months or Longer Gross Unrealized Losses		(66)
Held-to-Maturity, Total Estimated Fair Value	221,195	510,642
Held-to-Maturity, Total Gross Unrealized Losses	(460)	(9,268)
Non-GSE issuance REMICs and CMOs
Investments, Unrealized Loss Position [Line Items]
Available-for-Sale, Less Than Twelve Months Estimated Fair Value	534	276
Available-for-Sale,Less Than Twelve Months Gross Unrealized Losses	(7)	(2)
Available-for-Sale, Twelve Months or Longer Estimated Fair Value	17,651	19,991
Available-for-Sale, Twelve Months or Longer Gross Unrealized Losses	(317)	(360)
Available-for-Sale, Total Estimated Fair Value	18,185	20,267
Available-for-Sale, Total Gross Unrealized Losses	(324)	(362)
Held-to-Maturity, Twelve Months or Longer Estimated Fair Value		1,744
Held-to-Maturity-Twelve Months or Longer Gross Unrealized Losses		(66)
Held-to-Maturity, Total Estimated Fair Value		1,744
Held-to-Maturity, Total Gross Unrealized Losses		(66)
GSE pass-through certificates
Investments, Unrealized Loss Position [Line Items]
Available-for-Sale, Less Than Twelve Months Estimated Fair Value	824	1,775
Available-for-Sale,Less Than Twelve Months Gross Unrealized Losses	(5)	(2)
Available-for-Sale, Total Estimated Fair Value	824	1,775
Available-for-Sale, Total Gross Unrealized Losses	(5)	(2)
Freddie Mac and Fannie Mae stock
Investments, Unrealized Loss Position [Line Items]
Available-for-Sale, Twelve Months or Longer Gross Unrealized Losses	(15)	(15)
Available-for-Sale, Total Gross Unrealized Losses	(15)	(15)
GSE issuance REMICs and CMOs
Investments, Unrealized Loss Position [Line Items]
Held-to-Maturity, Less Than Twelve Months Estimated Fair Value	196,263	484,366
Held-to-Maturity, Less Than Twelve Months Gross Unrealized Losses	(392)	(8,734)
Held-to-Maturity, Total Estimated Fair Value	196,263	484,366
Held-to-Maturity, Total Gross Unrealized Losses	(392)	(8,734)
Obligations of GSEs
Investments, Unrealized Loss Position [Line Items]
Held-to-Maturity, Less Than Twelve Months Estimated Fair Value	24,932	24,532
Held-to-Maturity, Less Than Twelve Months Gross Unrealized Losses	(68)	(468)
Held-to-Maturity, Total Estimated Fair Value	24,932	24,532
Held-to-Maturity, Total Gross Unrealized Losses	$ (68)	$ (468)

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