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	12 Months Ended
	Dec. 31, 2014
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	13,533,147
Weighted- Average Yield	1.34%
One Year or Less - Carrying Value	1,069,561
One Year or Less - Weighted- Average Yield	1.48%
After One Year to Five Years - Carrying Value	8,683,092
After One Year to Five Years - Weighted- Average Yield	1.13%
After Five Years to Ten Years - Carrying Value	1,638,392
After Five Years to Ten Years - Weighted- Average Yield	2.27%
After Ten Years - Carrying Value	2,142,102
After Ten Years - Weighted- Average Yield	1.39%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	7,302,273
Weighted- Average Yield	1.08%
One Year or Less - Carrying Value	200,098
One Year or Less - Weighted- Average Yield	0.20%
After One Year to Five Years - Carrying Value	6,406,273
After One Year to Five Years - Weighted- Average Yield	1.02%
After Five Years to Ten Years - Carrying Value	695,902
After Five Years to Ten Years - Weighted- Average Yield	1.99%
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	3,561,556
Weighted- Average Yield	1.64%
One Year or Less - Carrying Value	869,463
One Year or Less - Weighted- Average Yield	1.78%
After One Year to Five Years - Carrying Value	2,276,819
After One Year to Five Years - Weighted- Average Yield	1.46%
After Five Years to Ten Years - Carrying Value	415,274
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]
Carrying Value	1,884,843
Weighted- Average Yield	2.00%
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	527,216
After Five Years to Ten Years - Weighted- Average Yield	2.57%
After Ten Years - Carrying Value	1,357,627
After Ten Years - Weighted- Average Yield	1.79%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	784,475
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	784,475
After Ten Years - Weighted- Average Yield	0.71%
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