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	Jun. 30, 2013
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 10,038,282
Weighted- Average Yield	1.69%
One Year or Less - Carrying Value	177,752
One Year or Less - Weighted- Average Yield	1.22%
After One Year to Five Years - Carrying Value	2,554,353
After One Year to Five Years - Weighted- Average Yield	1.52%
After Five Years to Ten Years - Carrying Value	1,871,788
After Five Years to Ten Years - Weighted- Average Yield	2.16%
After Ten Years - Carrying Value	5,434,389
After Ten Years - Weighted- Average Yield	1.63%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	24,999
Weighted- Average Yield	0.13%
One Year or Less - Carrying Value	24,999
One Year or Less - Weighted- Average Yield	0.13%
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	3,403,589
Weighted- Average Yield	1.54%
One Year or Less - Carrying Value	151,542
One Year or Less - Weighted- Average Yield	1.36%
After One Year to Five Years - Carrying Value	2,533,087
After One Year to Five Years - Weighted- Average Yield	1.49%
After Five Years to Ten Years - Carrying Value	718,960
After Five Years to Ten Years - Weighted- Average Yield	1.76%
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	1,214,035
Weighted- Average Yield	2.32%
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	1,107,136
After Five Years to Ten Years - Weighted- Average Yield	2.26%
After Ten Years - Carrying Value	106,899
After Ten Years - Weighted- Average Yield	2.95%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,468,682
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	3,468,682
After Ten Years - Weighted- Average Yield	1.92%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,438,772
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	1,438,772
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	401,204
Weighted- Average Yield	1.82%
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	401,204
After Ten Years - Weighted- Average Yield	1.82%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	87,001
Weighted- Average Yield	5.98%
One Year or Less - Carrying Value	1,211
One Year or Less - Weighted- Average Yield	5.49%
After One Year to Five Years - Carrying Value	21,266
After One Year to Five Years - Weighted- Average Yield	5.65%
After Five Years to Ten Years - Carrying Value	45,692
After Five Years to Ten Years - Weighted- Average Yield	6.03%
After Ten Years - Carrying Value	$ 18,832
After Ten Years - Weighted- Average Yield	6.26%