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, 2014
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 12,815,233
Weighted- Average Yield	1.90%
One Year or Less - Carrying Value	472,001
One Year or Less - Weighted- Average Yield	1.45%
After One Year to Five Years - Carrying Value	2,475,732
After One Year to Five Years - Weighted- Average Yield	1.61%
After Five Years to Ten Years - Carrying Value	3,047,868
After Five Years to Ten Years - Weighted- Average Yield	2.21%
After Ten Years - Carrying Value	6,819,632
After Ten Years - Weighted- Average Yield	1.90%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	683,519
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	99,063
After One Year to Five Years - Weighted- Average Yield	1.50%
After Five Years to Ten Years - Carrying Value	584,456
After Five Years to Ten Years - Weighted- Average Yield	1.96%
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	4,118,636
Weighted- Average Yield	1.71%
One Year or Less - Carrying Value	470,663
One Year or Less - Weighted- Average Yield	1.44%
After One Year to Five Years - Carrying Value	2,300,971
After One Year to Five Years - Weighted- Average Yield	1.55%
After Five Years to Ten Years - Carrying Value	1,347,002
After Five Years to Ten Years - Weighted- Average Yield	2.09%
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	3,021,019
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	47,706
After One Year to Five Years - Weighted- Average Yield	2.43%
After Five Years to Ten Years - Carrying Value	899,214
After Five Years to Ten Years - Weighted- Average Yield	2.24%
After Ten Years - Carrying Value	2,074,099
After Ten Years - Weighted- Average Yield	2.52%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,235,004
Weighted- Average Yield	1.91%
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	173,604
After Five Years to Ten Years - Weighted- Average Yield	2.92%
After Ten Years - Carrying Value	3,061,400
After Ten Years - Weighted- Average Yield	1.85%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,111,318
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	1,111,318
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	559,665
Weighted- Average Yield	2.19%
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	559,665
After Ten Years - Weighted- Average Yield	2.19%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	86,072
Weighted- Average Yield	5.99%
One Year or Less - Carrying Value	1,338
One Year or Less - Weighted- Average Yield	5.50%
After One Year to Five Years - Carrying Value	27,992
After One Year to Five Years - Weighted- Average Yield	5.74%
After Five Years to Ten Years - Carrying Value	43,592
After Five Years to Ten Years - Weighted- Average Yield	6.07%
After Ten Years - Carrying Value	$ 13,150
After Ten Years - Weighted- Average Yield	6.27%