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, 2015 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 15,302,983
Weighted- Average Yield	1.31%
One Year or Less - Carrying Value	$ 1,774,732
One Year or Less - Weighted- Average Yield	1.04%
After One Year to Five Years - Carrying Value	$ 10,777,733
After One Year to Five Years - Weighted- Average Yield	1.26%
After Five Years to Ten Years - Carrying Value	$ 1,157,931
After Five Years to Ten Years - Weighted- Average Yield	2.21%
After Ten Years - Carrying Value	$ 1,592,587
After Ten Years - Weighted- Average Yield	1.27%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 10,197,025
Weighted- Average Yield	1.16%
One Year or Less - Carrying Value	$ 953,659
One Year or Less - Weighted- Average Yield	0.45%
After One Year to Five Years - Carrying Value	$ 8,806,490
After One Year to Five Years - Weighted- Average Yield	1.20%
After Five Years to Ten Years - Carrying Value	$ 436,876
After Five Years to Ten Years - Weighted- Average Yield	1.75%
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	$ 2,942,968
Weighted- Average Yield	1.63%
One Year or Less - Carrying Value	$ 821,073
One Year or Less - Weighted- Average Yield	1.73%
After One Year to Five Years - Carrying Value	$ 1,971,243
After One Year to Five Years - Weighted- Average Yield	1.53%
After Five Years to Ten Years - Carrying Value	$ 150,652
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%
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	$ 1,517,702
Weighted- Average Yield	1.96%
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	$ 570,403
After Five Years to Ten Years - Weighted- Average Yield	2.50%
After Ten Years - Carrying Value	$ 947,299
After Ten Years - Weighted- Average Yield	1.65%
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	$ 645,288
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	$ 645,288
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