Investment Securities - Amortized Cost, Fair Value and Yield by Maturity Date (Detail) - USD ($) $ in Millions		9 Months Ended		12 Months Ended
		Sep. 30, 2021		Dec. 31, 2020
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 149,238	[1]	$ 133,611
Available-for-sale securities, total, fair value	[2]	$ 149,376	[1]	$ 136,840
Available-for-sale securities, total, weighted-average maturity in years		5 years 3 months 18 days	[1]	3 years 4 months 24 days
Available-for-sale securities, total, weighted-average yield		1.69%	[1],[3]	1.61%
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		$ 2,873
Available-for-sale securities, maturing after one year through five years, amortized cost		6,835
Available-for-sale securities, maturing after five years through ten years, amortized cost		7,146
Available-for-sale securities, maturing after ten years, amortized cost		1,606
Available-for-sale securities, Amortized Cost		18,460		$ 21,954
Available-for-sale securities, maturing in one year or less, fair value		2,894
Available-for-sale securities, maturing after one year through five years, fair value		6,977
Available-for-sale securities, maturing after five years through ten years, fair value		7,041
Available-for-sale securities, maturing after ten years, fair value		1,579
Available-for-sale securities, total, fair value		$ 18,491		22,391
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 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 10 months 24 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		12 years 7 months 6 days
Available-for-sale securities, total, weighted-average maturity in years		5 years 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.87%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	1.61%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3]	1.33%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3]	1.91%
Available-for-sale securities, total, weighted-average yield	[3]	1.57%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost	[4]	$ 140
Available-for-sale securities, maturing after one year through five years, amortized cost	[4]	54,361
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	66,370
Available-for-sale securities, maturing after ten years, amortized cost	[4]	16
Available-for-sale securities, Amortized Cost	[4]	120,887
Available-for-sale securities, maturing in one year or less, fair value	[4]	138
Available-for-sale securities, maturing after one year through five years, fair value	[4]	55,259
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	65,068
Available-for-sale securities, maturing after ten years, fair value	[4]	17
Available-for-sale securities, total, fair value	[4]	$ 120,482
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	6 months
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	3 years 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	6 years 6 months
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	12 years 2 months 12 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	5 years 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[4]	1.70%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[4]	1.57%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	1.53%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[4]	1.27%
Available-for-sale securities, total, weighted-average yield	[3],[4]	1.55%
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]	$ 63
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	1
Available-for-sale securities, Amortized Cost		64	[4]	200
Available-for-sale securities, maturing after one year through five years, fair value	[4]	66
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	2
Available-for-sale securities, maturing after ten years, fair value	[4]	1
Available-for-sale securities, total, fair value		$ 69	[4]	205
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	3 years 9 months 18 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	6 years 2 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	12 years 10 months 24 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	3 years 9 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[4]	2.69%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[4]	1.50%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	2.58%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[4]	2.41%
Available-for-sale securities, total, weighted-average yield	[3],[4]	1.51%
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]	$ 349
Available-for-sale securities, maturing after one year through five years, amortized cost	[5],[6]	2,492
Available-for-sale securities, maturing after five years through ten years, amortized cost	[5],[6]	5,595
Available-for-sale securities, maturing after ten years, amortized cost	[5],[6]	1,384
Available-for-sale securities, Amortized Cost		9,820	[5],[6]	8,166
Available-for-sale securities, maturing in one year or less, fair value	[5],[6]	355
Available-for-sale securities, maturing after one year through five years, fair value	[5],[6]	2,663
Available-for-sale securities, maturing after five years through ten years, fair value	[5],[6]	5,964
Available-for-sale securities, maturing after ten years, fair value	[5],[6]	1,345
Available-for-sale securities, total, fair value		$ 10,327	[5],[6]	8,861
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[5],[6]	4 years 1 month 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[5],[6]	6 years 10 months 24 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[5],[6]	17 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years	[5],[6]	7 years 4 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[5],[6]	4.28%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[5],[6]	4.24%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[5],[6]	3.73%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[5],[6]	2.50%
Available-for-sale securities, total, weighted-average yield	[3],[5],[6]	3.71%
Other [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 7
Available-for-sale securities, Amortized Cost		7		9
Available-for-sale securities, maturing after one year through five years, fair value		7
Available-for-sale securities, total, fair value		$ 7		$ 9
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 8 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 8 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	2.07%
Available-for-sale securities, total, weighted-average yield	[3]	2.07%

[1] The weighted-average maturity of total available-for-sale investment securities was 3.4 years at December 31, 2020, with a corresponding weighted-average yield of 1.61 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.