Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 64,249	$ 61,817
Held-to-maturity securities, total, amortized cost		42,113	43,590
Available-for-sale securities, total, fair value	[1]	$ 64,912	$ 61,997
Held-to-maturity securities, total, weighted-average yield		1.94%	1.92%
Available-for-sale securities, total, weighted-average yield		2.19%	2.21%
Available-for-sale securities, total, weighted-average maturity in years		4 years 3 months 18 days	4 years 8 months 12 days
Amortized Cost of Investment Securities		$ 106,362	$ 105,407
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Held-to-maturity securities, total, fair value		$ 42,521	$ 43,493
Held-to-maturity securities, total, weighted-average maturity in years		3 years 9 months 18 days	4 years 2 months 12 days
U.S. Treasury and Agencies [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 375
Available-for-sale securities, maturing after one year through five years, amortized cost		3,392
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,894
Available-for-sale securities, maturing after ten years, amortized cost		1
Available-for-sale securities, total, amortized cost		7,662	$ 4,611
Available-for-sale securities, maturing in one year or less, fair value		375
Held-to-maturity securities, maturing after one year through five years, amortized cost		1,098
Available-for-sale securities, maturing after one year through five years, fair value		3,422
Held-to-maturity securities, maturing after five years through ten years, amortized cost		1,810
Available-for-sale securities, maturing after five years through ten years, fair value		3,970
Available-for-sale securities, maturing after ten years, fair value		2
Held-to-maturity securities, total, amortized cost		2,908	2,925
Available-for-sale securities, total, fair value		$ 7,769	4,596
Available-for-sale securities, maturing in one year or less, weighted-average yield		2.46%
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		8 months 12 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.42%
Available for sale securities, maturing after one year through five years, weighted-average yield		1.27%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		2 years 9 months 18 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.14%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.94%
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 7 months 6 days
Available-for-sale securities, maturing after ten years, weighted-average yield		4.15%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		11 years 4 months 24 days
Held-to-maturity securities, total, weighted-average yield		1.87%
Available-for-sale securities, total, weighted-average yield		1.67%
Available-for-sale securities, total, weighted-average maturity in years		4 years 7 months 6 days
Amortized Cost of Investment Securities		$ 10,570	$ 7,536
Amortized Cost of Investment Securities, Percentage		9.90%	7.20%
Held to maturity securities, maturing after one year through five years, fair value		$ 1,115
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 4 months 24 days
Held-to-maturity securities, maturing after five years through ten years, fair value		$ 1,850
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 8 months 12 days
Held-to-maturity securities, total, fair value		$ 2,965	$ 2,919
Held-to-maturity securities, total, weighted-average maturity in years		5 years 1 month 6 days
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 793
Available-for-sale securities, maturing after one year through five years, amortized cost		45,011
Available-for-sale securities, maturing after five years through ten years, amortized cost		4,002
Available-for-sale securities, maturing after ten years, amortized cost		355
Available-for-sale securities, total, amortized cost		50,161
Held-to-maturity securities, maturing in one year or less, amortized cost		465
Available-for-sale securities, maturing in one year or less, fair value		795
Held-to-maturity securities, maturing after one year through five years, amortized cost		36,473
Available-for-sale securities, maturing after one year through five years, fair value		45,424
Held-to-maturity securities, maturing after five years through ten years, amortized cost		2,173
Available-for-sale securities, maturing after five years through ten years, fair value		4,036
Held-to-maturity securities, maturing after ten years, amortized cost		50
Available-for-sale securities, maturing after ten years, fair value		352
Held-to-maturity securities, total, amortized cost		39,161
Available-for-sale securities, total, fair value		$ 50,607
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.92%
Available-for-sale securities, maturing in one year or less, weighted-average yield		1.82%
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		7 months 6 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.99%
Available for sale securities, maturing after one year through five years, weighted-average yield		1.81%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 10 months 24 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.31%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.71%
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 2 months 12 days
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.31%
Available-for-sale securities, maturing after ten years, weighted-average yield		1.49%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		12 years 3 months 18 days
Held-to-maturity securities, total, weighted-average yield		1.95%
Available-for-sale securities, total, weighted-average yield		1.80%
Available-for-sale securities, total, weighted-average maturity in years		4 years 1 month 6 days
Amortized Cost of Investment Securities		$ 89,322	$ 91,265
Held-to-maturity securities, maturing in one year or less, fair value		$ 467
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		8 months 12 days
Amortized Cost of Investment Securities, Percentage		84.00%	86.60%
Held to maturity securities, maturing after one year through five years, fair value		$ 36,799
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 7 months 6 days
Held-to-maturity securities, maturing after five years through ten years, fair value		$ 2,189
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 10 months 24 days
Held-to-maturity securities, maturing after ten years, fair value		$ 49
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		12 years 2 months 12 days
Held-to-maturity securities, total, fair value		$ 39,504
Held-to-maturity securities, total, weighted-average maturity in years		3 years 8 months 12 days
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 15
Available-for-sale securities, maturing after one year through five years, amortized cost		171
Available-for-sale securities, maturing after five years through ten years, amortized cost		354
Available-for-sale securities, total, amortized cost		540
Available-for-sale securities, maturing in one year or less, fair value		18
Held-to-maturity securities, maturing after one year through five years, amortized cost		6
Available-for-sale securities, maturing after one year through five years, fair value		173
Held-to-maturity securities, maturing after five years through ten years, amortized cost		2
Available-for-sale securities, maturing after five years through ten years, fair value		360
Held-to-maturity securities, maturing after ten years, amortized cost		1
Held-to-maturity securities, total, amortized cost		9
Available-for-sale securities, total, fair value		$ 551
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.05%
Available-for-sale securities, maturing in one year or less, weighted-average yield		6.88%
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		8 months 12 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.08%
Available for sale securities, maturing after one year through five years, weighted-average yield		3.08%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 6 months
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.22%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.87%
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 8 months 12 days
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.19%
Held-to-maturity securities, total, weighted-average yield		1.13%
Available-for-sale securities, total, weighted-average yield		3.05%
Available-for-sale securities, total, weighted-average maturity in years		4 years 10 months 24 days
Amortized Cost of Investment Securities		$ 549	$ 558
Held-to-maturity securities, maturing in one year or less, fair value		$ 1
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		2 months 12 days
Amortized Cost of Investment Securities, Percentage		0.50%	0.50%
Held to maturity securities, maturing after one year through five years, fair value		$ 8
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 4 months 24 days
Held-to-maturity securities, maturing after five years through ten years, fair value		$ 2
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 9 months 18 days
Held-to-maturity securities, maturing after ten years, fair value		$ 7
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		11 years 10 months 24 days
Held-to-maturity securities, total, fair value		$ 18
Held-to-maturity securities, total, weighted-average maturity in years		5 years 1 month 6 days
Obligations of State and Political Subdivisions [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 2,204
Available-for-sale securities, maturing after one year through five years, amortized cost		1,587
Available-for-sale securities, maturing after five years through ten years, amortized cost		1,125
Available-for-sale securities, maturing after ten years, amortized cost		126
Available-for-sale securities, total, amortized cost		5,042	$ 5,149
Available-for-sale securities, maturing in one year or less, fair value		2,240
Held-to-maturity securities, maturing after one year through five years, amortized cost		1
Available-for-sale securities, maturing after one year through five years, fair value		1,667
Held-to-maturity securities, maturing after five years through ten years, amortized cost		3
Available-for-sale securities, maturing after five years through ten years, fair value		1,157
Held-to-maturity securities, maturing after ten years, amortized cost		4
Available-for-sale securities, maturing after ten years, fair value		133
Held-to-maturity securities, total, amortized cost		8	8
Available-for-sale securities, total, fair value		$ 5,197	5,316
Held-to-maturity securities, maturing in one year or less, weighted-average yield		9.09%
Available-for-sale securities, maturing in one year or less, weighted-average yield		7.03%
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		6 months
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		8.19%
Available for sale securities, maturing after one year through five years, weighted-average yield		6.83%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		1 year 9 months 18 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		3.90%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		5.53%
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 8 months 12 days
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.33%
Available-for-sale securities, maturing after ten years, weighted-average yield		6.65%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		12 years 1 month 6 days
Held-to-maturity securities, total, weighted-average yield		3.46%
Available-for-sale securities, total, weighted-average yield		6.62%
Available-for-sale securities, total, weighted-average maturity in years		2 years 9 months 18 days
Amortized Cost of Investment Securities		$ 5,050	$ 5,157
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		6 months
Amortized Cost of Investment Securities, Percentage		4.80%	4.90%
Held to maturity securities, maturing after one year through five years, fair value		$ 1
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 8 months 12 days
Held-to-maturity securities, maturing after five years through ten years, fair value		$ 3
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		8 years 7 months 6 days
Held-to-maturity securities, maturing after ten years, fair value		$ 4
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		10 years 2 months 12 days
Held-to-maturity securities, total, fair value		$ 8	$ 8
Held-to-maturity securities, total, weighted-average maturity in years		8 years 10 months 24 days
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after ten years, amortized cost		$ 678
Available-for-sale securities, total, amortized cost		678
Held-to-maturity securities, maturing in one year or less, amortized cost		3
Held-to-maturity securities, maturing after one year through five years, amortized cost		24
Available-for-sale securities, maturing after ten years, fair value		586
Held-to-maturity securities, total, amortized cost		27
Available-for-sale securities, total, fair value		$ 586
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.67%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.44%
Available-for-sale securities, maturing after ten years, weighted-average yield		2.79%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		17 years 3 months 18 days
Held-to-maturity securities, total, weighted-average yield		1.47%
Available-for-sale securities, total, weighted-average yield		2.79%
Available-for-sale securities, total, weighted-average maturity in years		17 years 3 months 18 days
Amortized Cost of Investment Securities		$ 871	$ 891
Held-to-maturity securities, maturing in one year or less, fair value		$ 3
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		7 months 6 days
Amortized Cost of Investment Securities, Percentage		0.80%	0.80%
Held to maturity securities, maturing after one year through five years, fair value		$ 23
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 8 months 12 days
Held-to-maturity securities, total, fair value		$ 26
Held-to-maturity securities, total, weighted-average maturity in years		3 years 4 months 24 days
Other Investments Including Perpetual Preferred Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 166
Available-for-sale securities, total, fair value		$ 202
Available-for-sale securities, total, weighted-average yield		4.95%
Available-for-sale securities, total, weighted-average maturity in years		28 years 10 months 24 days

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.