Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Schedule of Investments [Line Items]
Available-for-Sale, Amortized Cost	$ 421,000		$ 540,124
Available-for-Sale, Gross Unrealized Gains	27,884		22,208
Available-for-Sale, Gross Unrealized Losses	(344)		(379)
Available-for-Sale, Estimated Fair Value	448,540		561,953
Held-to-Maturity, Amortized Cost	1,966,836		2,003,784
Held-to-Maturity, Gross Unrealized Gains	47,775		47,594
Held-to-Maturity, Gross Unrealized Losses	(460)		(9,268)
Held-to-Maturity, Estimated Fair Value	2,014,151		2,042,110
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Available-for-Sale, Amortized Cost	420,985		540,109
Available-for-Sale, Gross Unrealized Gains	17,124		20,048
Available-for-Sale, Gross Unrealized Losses	(329)		(364)
Available-for-Sale, Estimated Fair Value	437,780		559,793
Held-to-Maturity, Amortized Cost	1,910,822		1,974,785
Held-to-Maturity, Gross Unrealized Gains	47,498		47,594
Held-to-Maturity, Gross Unrealized Losses	(392)		(8,800)
Held-to-Maturity, Estimated Fair Value	1,957,928		2,013,579
Residential mortgage-backed securities | GSE issuance REMICs and CMOs
Schedule of Investments [Line Items]
Available-for-Sale, Amortized Cost	376,143	[1],[2]	490,302
Available-for-Sale, Gross Unrealized Gains	16,099	[1],[2]	18,931
Available-for-Sale, Estimated Fair Value	392,242	[1],[2]	509,233
Held-to-Maturity, Amortized Cost	1,887,220		1,933,650
Held-to-Maturity, Gross Unrealized Gains	47,271		47,191
Held-to-Maturity, Gross Unrealized Losses	(392)		(8,734)
Held-to-Maturity, Estimated Fair Value	1,934,099		1,972,107
Residential mortgage-backed securities | Non-GSE issuance REMICs and CMOs
Schedule of Investments [Line Items]
Available-for-Sale, Amortized Cost	18,611		21,023
Available-for-Sale, Gross Unrealized Gains	1		3
Available-for-Sale, Gross Unrealized Losses	(324)		(362)
Available-for-Sale, Estimated Fair Value	18,288		20,664
Held-to-Maturity, Amortized Cost	22,991		40,363
Held-to-Maturity, Gross Unrealized Gains	184		352
Held-to-Maturity, Gross Unrealized Losses			(66)
Held-to-Maturity, Estimated Fair Value	23,175		40,649
Residential mortgage-backed securities | GSE pass-through certificates
Schedule of Investments [Line Items]
Available-for-Sale, Amortized Cost	26,231		28,784
Available-for-Sale, Gross Unrealized Gains	1,024		1,114
Available-for-Sale, Gross Unrealized Losses	(5)		(2)
Available-for-Sale, Estimated Fair Value	27,250		29,896
Held-to-Maturity, Amortized Cost	611		772
Held-to-Maturity, Gross Unrealized Gains	43		51
Held-to-Maturity, Estimated Fair Value	654		823
Freddie Mac and Fannie Mae stock
Schedule of Investments [Line Items]
Available-for-Sale, Amortized Cost	15		15
Available-for-Sale, Gross Unrealized Gains	10,760		2,160
Available-for-Sale, Gross Unrealized Losses	(15)		(15)
Available-for-Sale, Estimated Fair Value	10,760		2,160
Obligations of GSEs
Schedule of Investments [Line Items]
Held-to-Maturity, Amortized Cost	52,866		25,000
Held-to-Maturity, Gross Unrealized Gains	210
Held-to-Maturity, Gross Unrealized Losses	(68)		(468)
Held-to-Maturity, Estimated Fair Value	53,008		24,532
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Held-to-Maturity, Amortized Cost	3,148		3,999
Held-to-Maturity, Gross Unrealized Gains	67
Held-to-Maturity, Estimated Fair Value	$ 3,215		$ 3,999

[1] Government-sponsored enterprise

[2] Real estate mortgage investment conduits and collateralized mortgage obligations