Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2017	Dec. 31, 2016
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 67,739	$ 66,985
Available-for-sale securities, total, fair value	[1]	$ 67,455	$ 66,284
Available-for-sale securities, total, weighted-average maturity in years		4 years 10 months 25 days	5 years 1 month 6 days
Available-for-sale securities, total, weighted-average yield		2.17%	2.06%
Held-to-maturity securities, total, amortized cost		$ 43,659	$ 42,991
Held-to-maturity securities, total, fair value		$ 43,384	$ 42,435
Held-to-maturity securities, total, weighted-average maturity in years		4 years 6 months	4 years 7 months 6 days
Held-to-maturity securities, total, weighted-average yield		2.08%	1.93%
Amortized Cost of Investment Securities		$ 111,398	$ 109,976
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
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		$ 4,257
Available-for-sale securities, maturing after one year through five years, amortized cost		12,658
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,717
Available-for-sale securities, maturing after ten years, amortized cost		1
Available-for-sale securities, total, amortized cost		20,633	$ 17,314
Available-for-sale securities, maturing in one year or less, fair value		4,249
Available-for-sale securities, maturing after one year through five years, fair value		12,594
Available-for-sale securities, maturing after five years through ten years, fair value		3,694
Available-for-sale securities, maturing after ten years, fair value		2
Available-for-sale securities, total, fair value		$ 20,539	17,127
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		6 months
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		2 years 10 months 25 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 9 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		10 years 2 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 10 months 25 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		0.83%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.37%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.88%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.15%
Available-for-sale securities, total, weighted-average yield		1.35%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 225
Held-to-maturity securities, maturing after one year through five years, amortized cost		954
Held-to-maturity securities, maturing after five years through ten years, amortized cost		4,261
Held-to-maturity securities, total, amortized cost		5,440	5,246
Held-to-maturity securities, maturing in one year or less, fair value		225
Held-to-maturity securities, maturing after one year through five years, fair value		959
Held-to-maturity securities, maturing after five years through ten years, fair value		4,201
Held-to-maturity securities, total, fair value		$ 5,385	5,126
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		1 month 6 days
Held-to-maturity 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 maturity in years		6 years 3 months 19 days
Held-to-maturity securities, total, weighted-average maturity in years		5 years 6 months
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.02%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.81%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.81%
Held-to-maturity securities, total, weighted-average yield		1.78%
Amortized Cost of Investment Securities		$ 26,073	$ 22,560
Amortized Cost of Investment Securities, Percentage		23.40%	20.50%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 121
Available-for-sale securities, maturing after one year through five years, amortized cost		21,776
Available-for-sale securities, maturing after five years through ten years, amortized cost		17,636
Available-for-sale securities, maturing after ten years, amortized cost		1,650
Available-for-sale securities, total, amortized cost		41,183
Available-for-sale securities, maturing in one year or less, fair value		124
Available-for-sale securities, maturing after one year through five years, fair value		21,724
Available-for-sale securities, maturing after five years through ten years, fair value		17,474
Available-for-sale securities, maturing after ten years, fair value		1,654
Available-for-sale securities, total, fair value		$ 40,976
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		4 years 3 months 19 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 10 months 25 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		12 years 1 month 6 days
Available-for-sale securities, total, weighted-average maturity in years		5 years 3 months 19 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		4.04%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.05%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.14%
Available-for-sale securities, maturing after ten years, weighted-average yield		2.17%
Available-for-sale securities, total, weighted-average yield		2.09%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 204
Held-to-maturity securities, maturing after one year through five years, amortized cost		26,557
Held-to-maturity securities, maturing after five years through ten years, amortized cost		11,285
Held-to-maturity securities, maturing after ten years, amortized cost		136
Held-to-maturity securities, total, amortized cost		38,182
Held-to-maturity securities, maturing in one year or less, fair value		205
Held-to-maturity securities, maturing after one year through five years, fair value		26,432
Held-to-maturity securities, maturing after five years through ten years, fair value		11,180
Held-to-maturity securities, maturing after ten years, fair value		136
Held-to-maturity securities, total, fair value		$ 37,953
Held-to-maturity 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 maturity in years		3 years 9 months 18 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 7 months 6 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		11 years 8 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years 3 months 19 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		3.01%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.07%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.23%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.08%
Held-to-maturity securities, total, weighted-average yield		2.12%
Amortized Cost of Investment Securities		$ 79,365	$ 81,698
Amortized Cost of Investment Securities, Percentage		71.30%	74.30%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 346
Available-for-sale securities, maturing after five years through ten years, amortized cost		84
Available-for-sale securities, total, amortized cost		430
Available-for-sale securities, maturing after one year through five years, fair value		350
Available-for-sale securities, maturing after five years through ten years, fair value		87
Available-for-sale securities, total, fair value		$ 437
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		4 years
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		4 years 3 months 19 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		3.23%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.78%
Available-for-sale securities, total, weighted-average yield		3.14%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 4
Held-to-maturity securities, maturing after five years through ten years, amortized cost		3
Held-to-maturity securities, total, amortized cost		7
Held-to-maturity securities, maturing in one year or less, fair value		2
Held-to-maturity securities, maturing after one year through five years, fair value		5
Held-to-maturity securities, maturing after five years through ten years, fair value		3
Held-to-maturity securities, maturing after ten years, fair value		5
Held-to-maturity securities, total, fair value		$ 15
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		9 months 18 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 10 months 25 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 10 months 25 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		16 years 9 months 18 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years 6 months
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.12%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.88%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.93%
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.76%
Held-to-maturity securities, total, weighted-average yield		1.91%
Amortized Cost of Investment Securities		$ 437	$ 483
Amortized Cost of Investment Securities, Percentage		0.40%	0.40%
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		$ 824
Available-for-sale securities, maturing after one year through five years, amortized cost		555
Available-for-sale securities, maturing after five years through ten years, amortized cost		2,787
Available-for-sale securities, maturing after ten years, amortized cost		1,303
Available-for-sale securities, total, amortized cost		5,469	$ 5,167
Available-for-sale securities, maturing in one year or less, fair value		828
Available-for-sale securities, maturing after one year through five years, fair value		583
Available-for-sale securities, maturing after five years through ten years, fair value		2,809
Available-for-sale securities, maturing after ten years, fair value		1,249
Available-for-sale securities, total, fair value		$ 5,469	5,039
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		2 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 1 month 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		8 years 7 months 6 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		18 years 10 months 25 days
Available-for-sale securities, total, weighted-average maturity in years		9 years 2 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		7.39%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		6.12%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		5.46%
Available-for-sale securities, maturing after ten years, weighted-average yield		5.05%
Available-for-sale securities, total, weighted-average yield		5.72%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 1
Held-to-maturity securities, maturing after five years through ten years, amortized cost		5
Held-to-maturity securities, total, amortized cost		6	6
Held-to-maturity securities, maturing after one year through five years, fair value		1
Held-to-maturity securities, maturing after five years through ten years, fair value		6
Held-to-maturity securities, total, fair value		$ 7	7
Held-to-maturity 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 maturity in years		8 years 6 months
Held-to-maturity securities, total, weighted-average maturity in years		8 years
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		8.18%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.78%
Held-to-maturity securities, total, weighted-average yield		3.37%
Amortized Cost of Investment Securities		$ 5,475	$ 5,173
Amortized Cost of Investment Securities, Percentage		4.90%	4.70%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 2
Held-to-maturity securities, maturing after one year through five years, amortized cost		22
Held-to-maturity securities, total, amortized cost		24
Held-to-maturity securities, maturing in one year or less, fair value		2
Held-to-maturity securities, maturing after one year through five years, fair value		22
Held-to-maturity securities, total, fair value		$ 24
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		3 months 19 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 1 month 6 days
Held-to-maturity securities, total, weighted-average maturity in years		2 years 9 months 18 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.68%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.00%
Held-to-maturity securities, total, weighted-average yield		1.97%
Amortized Cost of Investment Securities		$ 48	$ 62
Amortized Cost of Investment Securities, Percentage			0.10%
Other Investment [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		24	$ 27
Available-for-sale securities, total, fair value		$ 34	$ 36

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