Investment Securities - 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, 2013
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 10,203,356
Weighted- Average Yield	1.74%
One Year or Less - Carrying Value	249,458
One Year or Less - Weighted- Average Yield	1.36%
After One Year to Five Years - Carrying Value	2,632,356
After One Year to Five Years - Weighted- Average Yield	1.53%
After Five Years to Ten Years - Carrying Value	1,836,122
After Five Years to Ten Years - Weighted- Average Yield	2.18%
After Ten Years - Carrying Value	5,485,420
After Ten Years - Weighted- Average Yield	1.70%
U.S. agency debentures
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,624,820
Weighted- Average Yield	1.56%
One Year or Less - Carrying Value	248,124
One Year or Less - Weighted- Average Yield	1.34%
After One Year to Five Years - Carrying Value	2,607,969
After One Year to Five Years - Weighted- Average Yield	1.49%
After Five Years to Ten Years - Carrying Value	768,727
After Five Years to Ten Years - Weighted- Average Yield	1.87%
After Ten Years - Carrying Value	0
After Ten Years - Weighted- Average Yield	0.00%
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,518,698
Weighted- Average Yield	2.38%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	889
After One Year to Five Years - Weighted- Average Yield	7.50%
After Five Years to Ten Years - Carrying Value	1,024,091
After Five Years to Ten Years - Weighted- Average Yield	2.25%
After Ten Years - Carrying Value	493,718
After Ten Years - Weighted- Average Yield	2.63%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,247,477
Weighted- Average Yield	1.90%
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	3,247,477
After Ten Years - Weighted- Average Yield	1.90%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,289,850
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	1,289,850
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	435,932
Weighted- Average Yield	1.92%
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	435,932
After Ten Years - Weighted- Average Yield	1.92%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	86,579
Weighted- Average Yield	5.98%
One Year or Less - Carrying Value	1,334
One Year or Less - Weighted- Average Yield	5.44%
After One Year to Five Years - Carrying Value	23,498
After One Year to Five Years - Weighted- Average Yield	5.69%
After Five Years to Ten Years - Carrying Value	43,304
After Five Years to Ten Years - Weighted- Average Yield	6.03%
After Ten Years - Carrying Value	$ 18,443
After Ten Years - Weighted- Average Yield	6.26%