Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) - Jun. 30, 2015 - Available-for-sale Securities - USD ($) $ in Thousands	Total
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 14,491,820
Weighted- Average Yield	1.28%
One Year or Less - Carrying Value	$ 1,247,728
One Year or Less - Weighted- Average Yield	1.34%
After One Year to Five Years - Carrying Value	$ 10,025,733
After One Year to Five Years - Weighted- Average Yield	1.22%
After Five Years to Ten Years - Carrying Value	$ 1,482,100
After Five Years to Ten Years - Weighted- Average Yield	1.65%
After Ten Years - Carrying Value	$ 1,736,259
After Ten Years - Weighted- Average Yield	1.28%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 9,001,966
Weighted- Average Yield	1.07%
One Year or Less - Carrying Value	$ 400,332
One Year or Less - Weighted- Average Yield	0.32%
After One Year to Five Years - Carrying Value	$ 7,866,030
After One Year to Five Years - Weighted- Average Yield	1.14%
After Five Years to Ten Years - Carrying Value	$ 735,604
After Five Years to Ten Years - Weighted- Average Yield	0.75%
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	$ 3,154,953
Weighted- Average Yield	1.65%
One Year or Less - Carrying Value	$ 847,396
One Year or Less - Weighted- Average Yield	1.82%
After One Year to Five Years - Carrying Value	$ 2,159,703
After One Year to Five Years - Weighted- Average Yield	1.53%
After Five Years to Ten Years - Carrying Value	$ 147,854
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	$ 1,642,593
Weighted- Average Yield	1.98%
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	$ 598,642
After Five Years to Ten Years - Weighted- Average Yield	2.55%
After Ten Years - Carrying Value	$ 1,043,951
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	$ 692,308
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	$ 692,308
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