Investment Securities - Amortized Cost, Fair Value and Yield by Maturity Date (Detail) - USD ($) $ in Millions		9 Months Ended		12 Months Ended
		Sep. 30, 2020		Dec. 31, 2019
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 130,740	[1]	$ 122,117
Available-for-sale securities, total, fair value	[2]	$ 134,032	[1]	$ 122,613
Available-for-sale securities, total, weighted-average maturity in years		3 years	[1]	4 years 2 months 12 days
Available-for-sale securities, total, weighted-average yield		1.78%	[1],[3]	2.38%
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		$ 7,788
Available-for-sale securities, maturing after one year through five years, amortized cost		9,669
Available-for-sale securities, maturing after five years through ten years, amortized cost		4,110
Available-for-sale securities, maturing after ten years, amortized cost		462
Available-for-sale securities, Amortized Cost		22,029		$ 19,845
Available-for-sale securities, maturing in one year or less, fair value		7,828
Available-for-sale securities, maturing after one year through five years, fair value		9,962
Available-for-sale securities, maturing after five years through ten years, fair value		4,315
Available-for-sale securities, maturing after ten years, fair value		459
Available-for-sale securities, total, fair value		$ 22,564		19,839
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		4 months 24 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		7 years 8 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		12 years 9 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 10 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.41%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	1.51%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3]	1.64%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3]	1.64%
Available-for-sale securities, total, weighted-average yield	[3]	1.50%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost	[4]	$ 790
Available-for-sale securities, maturing after one year through five years, amortized cost	[4]	94,241
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	5,634
Available-for-sale securities, maturing after ten years, amortized cost	[4]	46
Available-for-sale securities, Amortized Cost	[4]	100,711
Available-for-sale securities, maturing in one year or less, fair value	[4]	797
Available-for-sale securities, maturing after one year through five years, fair value	[4]	96,219
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	5,828
Available-for-sale securities, maturing after ten years, fair value	[4]	47
Available-for-sale securities, total, fair value	[4]	$ 102,891
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	8 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	2 years 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	8 years 1 month 6 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	11 years 6 months
Available-for-sale securities, total, weighted-average maturity in years	[4]	2 years 8 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.74%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	1.68%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3]	1.44%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3]	1.23%
Available-for-sale securities, total, weighted-average yield	[3]	1.67%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost	[4]	$ 202
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	1
Available-for-sale securities, Amortized Cost	[4]	203
Available-for-sale securities, maturing after one year through five years, fair value	[4]	206
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	1
Available-for-sale securities, maturing after ten years, fair value	[4]	1
Available-for-sale securities, total, fair value	[4]	$ 208
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	4 months 24 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	4 years 1 month 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	5 years 3 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	14 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	4 years 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	2.69%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	2.26%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3]	2.02%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3]	2.41%
Available-for-sale securities, total, weighted-average yield	[3]	2.26%
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	[5],[6]	$ 91
Available-for-sale securities, maturing after one year through five years, amortized cost	[5],[6]	1,087
Available-for-sale securities, maturing after five years through ten years, amortized cost	[5],[6]	6,521
Available-for-sale securities, maturing after ten years, amortized cost	[5],[6]	85
Available-for-sale securities, Amortized Cost		7,784	[5],[6]	6,499
Available-for-sale securities, maturing in one year or less, fair value	[5],[6]	92
Available-for-sale securities, maturing after one year through five years, fair value	[5],[6]	1,151
Available-for-sale securities, maturing after five years through ten years, fair value	[5],[6]	7,025
Available-for-sale securities, maturing after ten years, fair value	[5],[6]	88
Available-for-sale securities, total, fair value		$ 8,356	[5],[6]	$ 6,814
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[5],[6]	7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[5],[6]	3 years 1 month 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[5],[6]	7 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[5],[6]	11 years
Available-for-sale securities, total, weighted-average maturity in years	[5],[6]	6 years 4 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[5],[6]	4.61%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[5],[6]	4.41%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[5],[6]	4.01%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[5],[6]	3.00%
Available-for-sale securities, total, weighted-average yield	[3],[5],[6]	4.07%
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		$ 13
Available-for-sale securities, Amortized Cost		13
Available-for-sale securities, maturing in one year or less, fair value		13
Available-for-sale securities, total, fair value		$ 13
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		3 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		3 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.59%
Available-for-sale securities, total, weighted-average yield	[3]	1.59%

[1] The weighted-average maturity of total available-for-sale investment securities was 4.2 years at December 31, 2019, with a corresponding weighted-average yield of 2.38 percent.

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

[3] Weighted-average yields for obligations of state and political subdivisions are presented on a fully-taxable equivalent basis based on a federal income tax rate of 21 percent. Yields on investment securities are computed based on amortized cost balances.

[4] Information related to asset and mortgage-backed securities included above is presented based upon weighted-average maturities that take into account anticipated future prepayments.

[5] Information related to obligations of state and political subdivisions is presented based upon yield to first optional call date if the security is purchased at a premium, and yield to maturity if the security is purchased at par or a discount.

[6] Maturity calculations for obligations of state and political subdivisions are based on the first optional call date for securities with a fair value above par and the contractual maturity date for securities with a fair value equal to or below par.