Summary of 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, 2012
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 11,039,813
Weighted- Average Yield	1.75%
One Year or Less - Carrying Value	77,389
One Year or Less - Weighted- Average Yield	1.54%
After One Year to Five Years - Carrying Value	2,670,513
After One Year to Five Years - Weighted- Average Yield	1.52%
After Five Years to Ten Years - Carrying Value	1,751,912
After Five Years to Ten Years - Weighted- Average Yield	2.39%
After Ten Years - Carrying Value	6,539,999
After Ten Years - Weighted- Average Yield	1.70%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	25,429
Weighted- Average Yield	2.39%
One Year or Less - Carrying Value	25,429
One Year or Less - Weighted- Average Yield	2.39%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	0
After Ten Years - Weighted- Average Yield	0.00%
U.S. agency debentures
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	2,936,150
Weighted- Average Yield	1.56%
One Year or Less - Carrying Value	51,027
One Year or Less - Weighted- Average Yield	1.05%
After One Year to Five Years - Carrying Value	2,653,180
After One Year to Five Years - Weighted- Average Yield	1.49%
After Five Years to Ten Years - Carrying Value	231,943
After Five Years to Ten Years - Weighted- Average Yield	2.43%
After Ten Years - Carrying Value	0
After Ten Years - Weighted- Average Yield	0.00%
Residential mortgage-backed securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,590,582
Weighted- Average Yield	2.34%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	1,470,437
After Five Years to Ten Years - Weighted- Average Yield	2.27%
After Ten Years - Carrying Value	120,145
After Ten Years - Weighted- Average Yield	3.29%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	4,168,125
Weighted- Average Yield	2.06%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	4,168,125
After Ten Years - Weighted- Average Yield	2.06%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,939,799
Weighted- Average Yield	0.70%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	1,939,799
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	279,507
Weighted- Average Yield	2.01%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	279,507
After Ten Years - Weighted- Average Yield	2.01%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	100,221
Weighted- Average Yield	4.99%
One Year or Less - Carrying Value	933
One Year or Less - Weighted- Average Yield	5.20%
After One Year to Five Years - Carrying Value	17,333
After One Year to Five Years - Weighted- Average Yield	5.60%
After Five Years to Ten Years - Carrying Value	49,532
After Five Years to Ten Years - Weighted- Average Yield	6.02%
After Ten Years - Carrying Value	$ 32,423
After Ten Years - Weighted- Average Yield	6.24%