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	9 Months Ended
	Sep. 30, 2016 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 12,662,975
Weighted- Average Yield	1.39%
One Year or Less - Carrying Value	$ 2,078,654
One Year or Less - Weighted- Average Yield	0.88%
After One Year to Five Years - Carrying Value	$ 8,987,545
After One Year to Five Years - Weighted- Average Yield	1.48%
After Five Years to Ten Years - Carrying Value	$ 755,958
After Five Years to Ten Years - Weighted- Average Yield	2.09%
After Ten Years - Carrying Value	$ 840,818
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	$ 8,939,627
Weighted- Average Yield	1.31%
One Year or Less - Carrying Value	$ 1,652,444
One Year or Less - Weighted- Average Yield	0.80%
After One Year to Five Years - Carrying Value	$ 7,287,183
After One Year to Five Years - Weighted- Average Yield	1.43%
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	$ 2,126,572
Weighted- Average Yield	1.60%
One Year or Less - Carrying Value	$ 426,210
One Year or Less - Weighted- Average Yield	1.17%
After One Year to Five Years - Carrying Value	$ 1,700,362
After One Year to Five Years - Weighted- Average Yield	1.71%
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	$ 1,092,289
Weighted- Average Yield	1.91%
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	$ 755,958
After Five Years to Ten Years - Weighted- Average Yield	2.09%
After Ten Years - Carrying Value	$ 336,331
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	$ 504,487
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	$ 504,487
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