Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 74,122	$ 67,381
Amortized Cost of Investment Securities		$ 120,603	$ 113,431
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Available-for-sale securities, total, fair value	[1]	$ 74,598	$ 66,115
Available-for-sale securities, total, weighted-average maturity in years		4 years	5 years 4 months 24 days
Available-for-sale securities, total, weighted-average yield		2.56%	2.57%
Held-to-maturity securities, Amortized Cost		$ 46,481	$ 46,050
Held-to-maturity securities, total, fair value		$ 46,638	$ 44,964
Held-to-maturity securities, total, weighted-average maturity in years		3 years 9 months 18 days	5 years 2 months 12 days
Held-to-maturity securities, total, weighted-average yield		2.42%	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		$ 2,544
Available-for-sale securities, maturing after one year through five years, amortized cost		11,608
Available-for-sale securities, maturing after five years through ten years, amortized cost		1,391
Available-for-sale securities, Amortized Cost		15,543	$ 19,604
Available-for-sale securities, maturing in one year or less, fair value		2,540
Amortized Cost of Investment Securities		19,945	$ 24,706
Available-for-sale securities, maturing after one year through five years, fair value		$ 11,628
Amortized Cost of Investment Securities, Percentage		16.60%	21.80%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 1,385
Available-for-sale securities, total, fair value		$ 15,553	$ 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 1 month 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		8 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 4 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		1.54%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.68%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.82%
Available-for-sale securities, total, weighted-average yield		1.67%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 3,897
Held-to-maturity securities, maturing after five years through ten years, amortized cost		505
Held-to-maturity securities, Amortized Cost		4,402	5,102
Held-to-maturity securities, maturing after one year through five years, fair value		3,913
Held-to-maturity securities, maturing after five years through ten years, fair value		517
Held-to-maturity securities, total, fair value		$ 4,430	4,961
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		6 years 2 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		3 years 10 months 24 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.70%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.43%
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		$ 26
Available-for-sale securities, maturing after one year through five years, amortized cost		41,747
Available-for-sale securities, maturing after five years through ten years, amortized cost		9,120
Available-for-sale securities, maturing after ten years, amortized cost		913
Available-for-sale securities, Amortized Cost		51,806
Available-for-sale securities, maturing in one year or less, fair value		26
Amortized Cost of Investment Securities		93,860	$ 81,464
Available-for-sale securities, maturing after one year through five years, fair value		$ 41,856
Amortized Cost of Investment Securities, Percentage		77.80%	71.80%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 9,128
Available-for-sale securities, maturing after ten years, fair value		917
Available-for-sale securities, total, fair value		$ 51,927
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		3 years 6 months
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 3 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		11 years 3 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		4 years 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		3.01%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.59%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.58%
Available-for-sale securities, maturing after ten years, weighted-average yield		3.06%
Available-for-sale securities, total, weighted-average yield		2.60%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 228
Held-to-maturity securities, maturing after one year through five years, amortized cost		36,288
Held-to-maturity securities, maturing after five years through ten years, amortized cost		5,428
Held-to-maturity securities, maturing after ten years, amortized cost		110
Held-to-maturity securities, Amortized Cost		42,054
Held-to-maturity securities, maturing in one year or less, fair value		228
Held-to-maturity securities, maturing after one year through five years, fair value		36,406
Held-to-maturity securities, maturing after five years through ten years, fair value		5,437
Held-to-maturity securities, maturing after ten years, fair value		110
Held-to-maturity securities, total, fair value		$ 42,181
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 6 months
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 8 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		3 years 9 months 18 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.19%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.48%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.54%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.95%
Held-to-maturity securities, total, weighted-average yield		2.49%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 377
Available-for-sale securities, Amortized Cost		377
Amortized Cost of Investment Securities		382	$ 402
Available-for-sale securities, maturing after one year through five years, fair value		$ 382
Amortized Cost of Investment Securities, Percentage		0.30%	0.40%
Available-for-sale securities, total, fair value		$ 382
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 2 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 2 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		3.37%
Available-for-sale securities, total, weighted-average yield		3.37%
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 3 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, maturing after ten years, weighted-average maturity in years		15 years 6 months
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		2.74%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.85%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.67%
Held-to-maturity securities, total, weighted-average yield		2.75%
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		$ 97
Available-for-sale securities, maturing after one year through five years, amortized cost		697
Available-for-sale securities, maturing after five years through ten years, amortized cost		5,555
Available-for-sale securities, maturing after ten years, amortized cost		46
Available-for-sale securities, Amortized Cost		6,395	$ 6,836
Available-for-sale securities, maturing in one year or less, fair value		98
Amortized Cost of Investment Securities		6,399	$ 6,842
Available-for-sale securities, maturing after one year through five years, fair value		$ 726
Amortized Cost of Investment Securities, Percentage		5.30%	6.00%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 5,861
Available-for-sale securities, maturing after ten years, fair value		50
Available-for-sale securities, total, fair value		$ 6,735	$ 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 3 months 18 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 3 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		11 years 3 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		6 years 9 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		5.71%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.50%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		4.27%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.52%
Available-for-sale securities, total, weighted-average yield		4.32%
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		3 months 18 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 4 months 24 days
Held-to-maturity securities, total, weighted-average maturity in years		6 years
Held-to-maturity securities, maturing in one year or less, weighted-average yield		7.33%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		6.38%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.95%
Held-to-maturity securities, total, weighted-average yield		2.45%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 1
Available-for-sale securities, Amortized Cost		1
Available-for-sale securities, maturing in one year or less, fair value		1
Amortized Cost of Investment Securities		17	$ 17
Available-for-sale securities, total, fair value		$ 1
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		1 month 6 days
Available-for-sale securities, total, weighted-average maturity in years		1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		2.55%
Available-for-sale securities, total, weighted-average yield		2.55%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 16
Held-to-maturity securities, Amortized Cost		16
Held-to-maturity securities, maturing in one year or less, fair value		16
Held-to-maturity securities, total, fair value		$ 16
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		4 months 24 days
Held-to-maturity securities, total, weighted-average maturity in years		4 months 24 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		3.12%
Held-to-maturity securities, total, weighted-average yield		3.12%

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