Summary of the Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2011
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 9,638,868
Weighted- Average Yield	1.84%
One Year or Less - Carrying Value	63,985
One Year or Less - Weighted- Average Yield	2.54%
After One Year to Five Years - Carrying Value	2,842,991
After One Year to Five Years - Weighted- Average Yield	1.52%
After Five Years to Ten Years - Carrying Value	1,491,303
After Five Years to Ten Years - Weighted- Average Yield	2.71%
After Ten Years - Carrying Value	5,240,589
After Ten Years - Weighted- Average Yield	1.77%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	26,130
Weighted- Average Yield	2.39%
After One Year to Five Years - Carrying Value	26,130
After One Year to Five Years - Weighted- Average Yield	2.39%
U.S. agency debentures
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	2,945,537
Weighted- Average Yield	1.55%
One Year or Less - Carrying Value	63,421
One Year or Less - Weighted- Average Yield	2.51%
After One Year to Five Years - Carrying Value	2,804,770
After One Year to Five Years - Weighted- Average Yield	1.49%
After Five Years to Ten Years - Carrying Value	77,346
After Five Years to Ten Years - Weighted- Average Yield	3.28%
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,503,851
Weighted- Average Yield	2.66%
After Five Years to Ten Years - Carrying Value	1,369,093
After Five Years to Ten Years - Weighted- Average Yield	2.57%
After Ten Years - Carrying Value	134,758
After Ten Years - Weighted- Average Yield	3.58%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	2,402,232
Weighted- Average Yield	2.72%
After Ten Years - Carrying Value	2,402,232
After Ten Years - Weighted- Average Yield	2.72%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	2,559,161
Weighted- Average Yield	0.70%
After Ten Years - Carrying Value	2,559,161
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	103,181
Weighted- Average Yield	2.22%
After Ten Years - Carrying Value	103,181
After Ten Years - Weighted- Average Yield	2.22%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	98,776
Weighted- Average Yield	6.00%
One Year or Less - Carrying Value	564
One Year or Less - Weighted- Average Yield	5.38%
After One Year to Five Years - Carrying Value	12,091
After One Year to Five Years - Weighted- Average Yield	5.48%
After Five Years to Ten Years - Carrying Value	44,864
After Five Years to Ten Years - Weighted- Average Yield	5.97%
After Ten Years - Carrying Value	$ 41,257
After Ten Years - Weighted- Average Yield	6.20%