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	6 Months Ended
	Jun. 30, 2016 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 13,057,691
Weighted- Average Yield	1.40%
One Year or Less - Carrying Value	$ 1,729,960
One Year or Less - Weighted- Average Yield	0.92%
After One Year to Five Years - Carrying Value	$ 9,583,740
After One Year to Five Years - Weighted- Average Yield	1.45%
After Five Years to Ten Years - Carrying Value	$ 848,402
After Five Years to Ten Years - Weighted- Average Yield	2.11%
After Ten Years - Carrying Value	$ 895,589
After Ten Years - Weighted- Average Yield	1.03%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 8,977,098
Weighted- Average Yield	1.31%
One Year or Less - Carrying Value	$ 1,353,468
One Year or Less - Weighted- Average Yield	0.78%
After One Year to Five Years - Carrying Value	$ 7,623,630
After One Year to Five Years - Weighted- Average Yield	1.41%
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	$ 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	$ 2,336,602
Weighted- Average Yield	1.59%
One Year or Less - Carrying Value	$ 376,492
One Year or Less - Weighted- Average Yield	1.42%
After One Year to Five Years - Carrying Value	$ 1,960,110
After One Year to Five Years - Weighted- Average Yield	1.62%
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	$ 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	$ 1,206,294
Weighted- Average Yield	1.93%
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	$ 848,402
After Five Years to Ten Years - Weighted- Average Yield	2.11%
After Ten Years - Carrying Value	$ 357,892
After Ten Years - Weighted- Average Yield	1.52%
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	$ 537,697
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	$ 537,697
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