Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2019	Dec. 31, 2018
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 68,980	$ 67,381
Amortized Cost of Investment Securities		$ 115,363	$ 113,431
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Available-for-sale securities, total, fair value	[1]	$ 69,197	$ 66,115
Available-for-sale securities, total, weighted-average maturity in years		4 years 1 month 6 days	5 years 4 months 24 days
Available-for-sale securities, total, weighted-average yield		2.65%	2.57%
Held-to-maturity securities, Amortized Cost		$ 46,383	$ 46,050
Held-to-maturity securities, total, fair value		$ 46,350	$ 44,964
Held-to-maturity securities, total, weighted-average maturity in years		4 years	5 years 2 months 12 days
Held-to-maturity securities, total, weighted-average yield		2.50%	2.46%
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		$ 2,034
Available-for-sale securities, maturing after one year through five years, amortized cost		14,920
Available-for-sale securities, maturing after five years through ten years, amortized cost		220
Available-for-sale securities, Amortized Cost		17,174	$ 19,604
Available-for-sale securities, maturing in one year or less, fair value		2,027
Amortized Cost of Investment Securities		21,822	$ 24,706
Available-for-sale securities, maturing after one year through five years, fair value		$ 14,919
Amortized Cost of Investment Securities, Percentage		18.90%	21.80%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 225
Available-for-sale securities, total, fair value		$ 17,171	$ 19,257
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		2 years 6 months
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 2 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 3 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		1.60%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.73%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.84%
Available-for-sale securities, total, weighted-average yield		1.73%
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		3,902
Held-to-maturity securities, maturing after five years through ten years, amortized cost		521
Held-to-maturity securities, Amortized Cost		4,648	5,102
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		3,897
Held-to-maturity securities, maturing after five years through ten years, fair value		527
Held-to-maturity securities, total, fair value		$ 4,649	4,961
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		3 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 18 days
Held-to-maturity securities, total, weighted-average maturity in years		3 years 10 months 24 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.74%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.71%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.44%
Held-to-maturity securities, total, weighted-average yield		1.79%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 29
Available-for-sale securities, maturing after one year through five years, amortized cost		33,720
Available-for-sale securities, maturing after five years through ten years, amortized cost		9,560
Available-for-sale securities, maturing after ten years, amortized cost		1,841
Available-for-sale securities, Amortized Cost		45,150
Available-for-sale securities, maturing in one year or less, fair value		29
Amortized Cost of Investment Securities		86,859	$ 81,464
Available-for-sale securities, maturing after one year through five years, fair value		$ 33,772
Amortized Cost of Investment Securities, Percentage		75.30%	71.80%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 9,487
Available-for-sale securities, maturing after ten years, fair value		1,847
Available-for-sale securities, total, fair value		$ 45,135
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		8 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 6 months
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		11 years 10 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		4 years 4 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		3.09%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.74%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.62%
Available-for-sale securities, maturing after ten years, weighted-average yield		3.42%
Available-for-sale securities, total, weighted-average yield		2.75%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 199
Held-to-maturity securities, maturing after one year through five years, amortized cost		33,145
Held-to-maturity securities, maturing after five years through ten years, amortized cost		8,136
Held-to-maturity securities, maturing after ten years, amortized cost		229
Held-to-maturity securities, Amortized Cost		41,709
Held-to-maturity securities, maturing in one year or less, fair value		198
Held-to-maturity securities, maturing after one year through five years, fair value		33,133
Held-to-maturity securities, maturing after five years through ten years, fair value		8,111
Held-to-maturity securities, maturing after ten years, fair value		231
Held-to-maturity securities, total, fair value		$ 41,673
Held-to-maturity 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 maturity in years		3 years 7 months 6 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 6 months
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		12 years 1 month 6 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.20%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.56%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.66%
Held-to-maturity securities, maturing after ten years, weighted-average yield		3.37%
Held-to-maturity securities, total, weighted-average yield		2.58%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 383
Available-for-sale securities, Amortized Cost		383
Amortized Cost of Investment Securities		388	$ 402
Available-for-sale securities, maturing after one year through five years, fair value		$ 388
Amortized Cost of Investment Securities, Percentage		0.30%	0.40%
Available-for-sale securities, total, fair value		$ 388
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 3 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 3 months 18 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		3.70%
Available-for-sale securities, total, weighted-average yield		3.70%
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		1
Held-to-maturity securities, Amortized Cost		5
Held-to-maturity securities, maturing after one year through five years, fair value		6
Held-to-maturity securities, maturing after ten years, fair value		1
Held-to-maturity securities, total, fair value		$ 7
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, weighted-average maturity in years		6 years 3 months 18 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		15 years 7 months 6 days
Held-to-maturity securities, total, weighted-average maturity in years		3 years 8 months 12 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		3.12%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		3.21%
Held-to-maturity securities, maturing after ten years, weighted-average yield		3.02%
Held-to-maturity securities, total, weighted-average yield		3.13%
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		$ 189
Available-for-sale securities, maturing after one year through five years, amortized cost		686
Available-for-sale securities, maturing after five years through ten years, amortized cost		5,202
Available-for-sale securities, maturing after ten years, amortized cost		196
Available-for-sale securities, Amortized Cost		6,273	$ 6,836
Available-for-sale securities, maturing in one year or less, fair value		190
Amortized Cost of Investment Securities		6,277	$ 6,842
Available-for-sale securities, maturing after one year through five years, fair value		$ 710
Amortized Cost of Investment Securities, Percentage		5.50%	6.00%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 5,406
Available-for-sale securities, maturing after ten years, fair value		197
Available-for-sale securities, total, fair value		$ 6,503	$ 6,701
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 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 6 months
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		22 years 3 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		7 years 3 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		5.67%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.55%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		4.32%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.30%
Available-for-sale securities, total, weighted-average yield		4.39%
Held-to-maturity securities, maturing after five years through ten years, amortized cost		$ 4
Held-to-maturity securities, Amortized Cost		4	6
Held-to-maturity securities, maturing after five years through ten years, fair value		4
Held-to-maturity securities, total, fair value		$ 4	7
Held-to-maturity 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 maturity in years		3 years 1 month 6 days
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, weighted-average maturity in years		6 years 2 months 12 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		7.34%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		6.40%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.89%
Held-to-maturity securities, total, weighted-average yield		2.40%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Amortized Cost of Investment Securities		$ 17	$ 17
Held-to-maturity securities, maturing in one year or less, amortized cost		17
Held-to-maturity securities, Amortized Cost		17
Held-to-maturity securities, maturing in one year or less, fair value		17
Held-to-maturity securities, total, fair value		$ 17
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		6 months
Held-to-maturity securities, total, weighted-average maturity in years		6 months
Held-to-maturity securities, maturing in one year or less, weighted-average yield		3.30%
Held-to-maturity securities, total, weighted-average yield		3.30%

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