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	Mar. 31, 2013
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 10,899,537
Weighted- Average Yield	1.69%
One Year or Less - Carrying Value	252,351
One Year or Less - Weighted- Average Yield	1.07%
After One Year to Five Years - Carrying Value	2,603,707
After One Year to Five Years - Weighted- Average Yield	1.51%
After Five Years to Ten Years - Carrying Value	2,026,029
After Five Years to Ten Years - Weighted- Average Yield	2.17%
After Ten Years - Carrying Value	6,017,450
After Ten Years - Weighted- Average Yield	1.64%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	100,046
Weighted- Average Yield	0.68%
One Year or Less - Carrying Value	100,046
One Year or Less - Weighted- Average Yield	0.68%
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	3,483,903
Weighted- Average Yield	1.54%
One Year or Less - Carrying Value	151,367
One Year or Less - Weighted- Average Yield	1.30%
After One Year to Five Years - Carrying Value	2,584,574
After One Year to Five Years - Weighted- Average Yield	1.48%
After Five Years to Ten Years - Carrying Value	747,962
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	1,347,830
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	1,228,081
After Five Years to Ten Years - Weighted- Average Yield	2.26%
After Ten Years - Carrying Value	119,749
After Ten Years - Weighted- Average Yield	2.98%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,855,067
Weighted- Average Yield	1.94%
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	3,855,067
After Ten Years - Weighted- Average Yield	1.94%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,602,853
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	1,602,853
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	419,335
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	419,335
After Ten Years - Weighted- Average Yield	1.82%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	90,503
Weighted- Average Yield	5.98%
One Year or Less - Carrying Value	938
One Year or Less - Weighted- Average Yield	5.32%
After One Year to Five Years - Carrying Value	19,133
After One Year to Five Years - Weighted- Average Yield	5.65%
After Five Years to Ten Years - Carrying Value	49,986
After Five Years to Ten Years - Weighted- Average Yield	6.02%
After Ten Years - Carrying Value	$ 20,446
After Ten Years - Weighted- Average Yield	6.25%