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	3 Months Ended
	Mar. 31, 2016 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 14,324,836
Weighted- Average Yield	1.39%
One Year or Less - Carrying Value	$ 1,034,252
One Year or Less - Weighted- Average Yield	1.04%
After One Year to Five Years - Carrying Value	$ 11,405,533
After One Year to Five Years - Weighted- Average Yield	1.40%
After Five Years to Ten Years - Carrying Value	$ 884,785
After Five Years to Ten Years - Weighted- Average Yield	2.15%
After Ten Years - Carrying Value	$ 1,000,266
After Ten Years - Weighted- Average Yield	1.06%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 9,954,354
Weighted- Average Yield	1.31%
One Year or Less - Carrying Value	$ 651,416
One Year or Less - Weighted- Average Yield	0.73%
After One Year to Five Years - Carrying Value	$ 9,302,938
After One Year to Five Years - Weighted- Average Yield	1.35%
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	$ 2,485,431
Weighted- Average Yield	1.60%
One Year or Less - Carrying Value	$ 382,836
One Year or Less - Weighted- Average Yield	1.57%
After One Year to Five Years - Carrying Value	$ 2,102,595
After One Year to Five Years - Weighted- Average Yield	1.60%
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	$ 1,312,017
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	$ 884,785
After Five Years to Ten Years - Weighted- Average Yield	2.15%
After Ten Years - Carrying Value	$ 427,232
After Ten Years - Weighted- Average Yield	1.53%
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	$ 573,034
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	$ 573,034
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