Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) (Available-for-sale Securities, USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 13,324,022
Weighted- Average Yield	1.35%
One Year or Less - Carrying Value	795,388
One Year or Less - Weighted- Average Yield	1.45%
After One Year to Five Years - Carrying Value	8,267,147
After One Year to Five Years - Weighted- Average Yield	1.13%
After Five Years to Ten Years - Carrying Value	1,903,070
After Five Years to Ten Years - Weighted- Average Yield	2.14%
After Ten Years - Carrying Value	2,358,417
After Ten Years - Weighted- Average Yield	1.43%
Lower Limit
Investments Classified by Contractual Maturity Date [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Upper Limit
Investments Classified by Contractual Maturity Date [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	6,876,299
Weighted- Average Yield	1.08%
One Year or Less - Carrying Value	100,189
One Year or Less - Weighted- Average Yield	0.15%
After One Year to Five Years - Carrying Value	6,041,278
After One Year to Five Years - Weighted- Average Yield	0.99%
After Five Years to Ten Years - Carrying Value	734,832
After Five Years to Ten Years - Weighted- Average Yield	1.97%
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]
Available-for-sale Securities, Debt Securities	3,605,833
Weighted- Average Yield	1.63%
One Year or Less - Carrying Value	695,199
One Year or Less - Weighted- Average Yield	1.64%
After One Year to Five Years - Carrying Value	2,225,869
After One Year to Five Years - Weighted- Average Yield	1.52%
After Five Years to Ten Years - Carrying Value	684,765
After Five Years to Ten Years - Weighted- Average Yield	2.01%
After Ten Years - Carrying Value	0
After Ten Years - Weighted- Average Yield	0.00%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	2,005,933
Weighted- Average Yield	2.01%
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	483,473
After Five Years to Ten Years - Weighted- Average Yield	2.59%
After Ten Years - Carrying Value	1,522,460
After Ten Years - Weighted- Average Yield	1.83%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	835,957
Weighted- Average Yield	0.71%
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	$ 835,957
After Ten Years - Weighted- Average Yield	0.71%