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	3 Months Ended
	Mar. 31, 2015
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	13,745,147
Weighted- Average Yield	1.33%
One Year or Less - Carrying Value	1,185,702
One Year or Less - Weighted- Average Yield	1.34%
After One Year to Five Years - Carrying Value	8,980,675
After One Year to Five Years - Weighted- Average Yield	1.17%
After Five Years to Ten Years - Carrying Value	1,712,666
After Five Years to Ten Years - Weighted- Average Yield	2.27%
After Ten Years - Carrying Value	1,866,104
After Ten Years - Weighted- Average Yield	1.28%
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	7,914,125
Weighted- Average Yield	1.11%
One Year or Less - Carrying Value	350,203
One Year or Less - Weighted- Average Yield	0.25%
After One Year to Five Years - Carrying Value	6,925,629
After One Year to Five Years - Weighted- Average Yield	1.09%
After Five Years to Ten Years - Carrying Value	638,293
After Five Years to Ten Years - Weighted- Average Yield	1.90%
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	3,313,664
Weighted- Average Yield	1.65%
One Year or Less - Carrying Value	835,499
One Year or Less - Weighted- Average Yield	1.80%
After One Year to Five Years - Carrying Value	2,055,046
After One Year to Five Years - Weighted- Average Yield	1.44%
After Five Years to Ten Years - Carrying Value	423,119
After Five Years to Ten Years - Weighted- Average Yield	2.36%
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	1,778,155
Weighted- Average Yield	1.99%
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	651,254
After Five Years to Ten Years - Weighted- Average Yield	2.58%
After Ten Years - Carrying Value	1,126,901
After Ten Years - Weighted- Average Yield	1.65%
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	739,203
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	739,203
After Ten Years - Weighted- Average Yield	0.71%