Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 11,661,786
Weighted- Average Yield	1.41%
One Year or Less - Carrying Value	640,289
One Year or Less - Weighted- Average Yield	1.36%
After One Year to Five Years - Carrying Value	6,569,032
After One Year to Five Years - Weighted- Average Yield	1.19%
After Five Years to Ten Years - Carrying Value	1,799,648
After Five Years to Ten Years - Weighted- Average Yield	2.10%
After Ten Years - Carrying Value	2,652,817
After Ten Years - Weighted- Average Yield	1.48%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	4,882,172
Weighted- Average Yield	1.11%
One Year or Less - Carrying Value	50,062
One Year or Less - Weighted- Average Yield	0.12%
After One Year to Five Years - Carrying Value	4,337,387
After One Year to Five Years - Weighted- Average Yield	1.01%
After Five Years to Ten Years - Carrying Value	494,723
After Five Years to Ten Years - Weighted- Average Yield	2.03%
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	3,745,873
Weighted- Average Yield	1.62%
One Year or Less - Carrying Value	590,227
One Year or Less - Weighted- Average Yield	1.47%
After One Year to Five Years - Carrying Value	2,231,645
After One Year to Five Years - Weighted- Average Yield	1.55%
After Five Years to Ten Years - Carrying Value	924,001
After Five Years to Ten Years - Weighted- Average Yield	1.89%
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	2,142,829
Weighted- Average Yield	2.01%
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	380,924
After Five Years to Ten Years - Weighted- Average Yield	2.69%
After Ten Years - Carrying Value	1,761,905
After Ten Years - Weighted- Average Yield	1.87%
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	890,912
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	$ 890,912
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