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 $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 16,375,279
Weighted- Average Yield	1.34%
One Year or Less - Carrying Value	$ 2,043,469
One Year or Less - Weighted- Average Yield	0.84%
After One Year to Five Years - Carrying Value	$ 11,918,727
After One Year to Five Years - Weighted- Average Yield	1.34%
After Five Years to Ten Years - Carrying Value	$ 1,176,232
After Five Years to Ten Years - Weighted- Average Yield	2.36%
After Ten Years - Carrying Value	$ 1,236,851
After Ten Years - Weighted- Average Yield	1.14%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 11,678,035
Weighted- Average Yield	1.24%
One Year or Less - Carrying Value	$ 1,451,224
One Year or Less - Weighted- Average Yield	0.54%
After One Year to Five Years - Carrying Value	$ 9,870,288
After One Year to Five Years - Weighted- Average Yield	1.29%
After Five Years to Ten Years - Carrying Value	$ 356,523
After Five Years to Ten Years - Weighted- Average Yield	2.49%
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	$ 2,690,029
Weighted- Average Yield	1.60%
One Year or Less - Carrying Value	$ 592,245
One Year or Less - Weighted- Average Yield	1.60%
After One Year to Five Years - Carrying Value	$ 2,048,439
After One Year to Five Years - Weighted- Average Yield	1.57%
After Five Years to Ten Years - Carrying Value	$ 49,345
After Five Years to Ten Years - Weighted- Average Yield	2.65%
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	$ 1,399,279
Weighted- Average Yield	1.95%
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	$ 770,364
After Five Years to Ten Years - Weighted- Average Yield	2.28%
After Ten Years - Carrying Value	$ 628,915
After Ten Years - Weighted- Average Yield	1.56%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 607,936
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	$ 607,936
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