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	Dec. 31, 2013
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 11,981,770
Weighted- Average Yield	1.98%
One Year or Less - Carrying Value	198,627
One Year or Less - Weighted- Average Yield	1.29%
After One Year to Five Years - Carrying Value	2,952,872
After One Year to Five Years - Weighted- Average Yield	1.54%
After Five Years to Ten Years - Carrying Value	2,260,721
After Five Years to Ten Years - Weighted- Average Yield	2.20%
After Ten Years - Carrying Value	6,569,550
After Ten Years - Weighted- Average Yield	1.87%
U.S. agency debentures
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	4,345,232
Weighted- Average Yield	2.03%
One Year or Less - Carrying Value	197,305
One Year or Less - Weighted- Average Yield	1.26%
After One Year to Five Years - Carrying Value	2,926,457
After One Year to Five Years - Weighted- Average Yield	1.50%
After Five Years to Ten Years - Carrying Value	1,221,470
After Five Years to Ten Years - Weighted- Average Yield	2.00%
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	2,473,576
Weighted- Average Yield	2.44%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	876
After One Year to Five Years - Weighted- Average Yield	7.50%
After Five Years to Ten Years - Carrying Value	953,783
After Five Years to Ten Years - Weighted- Average Yield	2.24%
After Ten Years - Carrying Value	1,518,917
After Ten Years - Weighted- Average Yield	2.56%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,325,758
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	41,945
After Five Years to Ten Years - Weighted- Average Yield	2.81%
After Ten Years - Carrying Value	3,283,813
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	1,186,573
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	1,186,573
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	564,604
Weighted- Average Yield	2.18%
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	564,604
After Ten Years - Weighted- Average Yield	2.18%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	86,027
Weighted- Average Yield	5.98%
One Year or Less - Carrying Value	1,322
One Year or Less - Weighted- Average Yield	5.44%
After One Year to Five Years - Carrying Value	25,539
After One Year to Five Years - Weighted- Average Yield	5.71%
After Five Years to Ten Years - Carrying Value	43,523
After Five Years to Ten Years - Weighted- Average Yield	6.05%
After Ten Years - Carrying Value	$ 15,643
After Ten Years - Weighted- Average Yield	6.26%