Investment Securities - Amortized Cost, Fair Value and Yield by Maturity Date (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2021		Dec. 31, 2020
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 156,175	[1]	$ 133,611
Available-for-sale securities, total, fair value	[2]	$ 156,003	[1]	$ 136,840
Available-for-sale securities, total, weighted-average maturity in years		5 years 10 months 24 days	[1]	3 years 4 months 24 days
Available-for-sale securities, total, weighted-average yield		1.55%	[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		$ 4,264
Available-for-sale securities, maturing after one year through five years, amortized cost		11,534
Available-for-sale securities, maturing after five years through ten years, amortized cost		7,364
Available-for-sale securities, maturing after ten years, amortized cost		1,239
Available-for-sale securities, Amortized Cost		24,401		$ 21,954
Available-for-sale securities, maturing in one year or less, fair value		4,292
Available-for-sale securities, maturing after one year through five years, fair value		11,694
Available-for-sale securities, maturing after five years through ten years, fair value		7,150
Available-for-sale securities, maturing after ten years, fair value		1,181
Available-for-sale securities, total, fair value		$ 24,317		22,391
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 7 months 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		8 years 1 month 6 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		13 years
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]	1.54%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	1.15%
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]	1.78%
Available-for-sale securities, total, weighted-average yield	[3]	1.31%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost	[4]	$ 110
Available-for-sale securities, maturing after one year through five years, amortized cost	[4]	47,936
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	74,785
Available-for-sale securities, maturing after ten years, amortized cost	[4]	52
Available-for-sale securities, Amortized Cost	[4]	122,883
Available-for-sale securities, maturing in one year or less, fair value	[4]	112
Available-for-sale securities, maturing after one year through five years, fair value	[4]	49,021
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	73,077
Available-for-sale securities, maturing after ten years, fair value	[4]	52
Available-for-sale securities, total, fair value	[4]	$ 122,262
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 7 months 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	7 years 9 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	11 years 10 months 24 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	6 years 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[4]	2.07%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[4]	1.40%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	1.46%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[4]	1.13%
Available-for-sale securities, total, weighted-average yield	[3],[4]	1.44%
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]	$ 3
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	194
Available-for-sale securities, Amortized Cost		197	[4]	200
Available-for-sale securities, maturing after one year through five years, fair value	[4]	4
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	197
Available-for-sale securities, maturing after ten years, fair value	[4]	1
Available-for-sale securities, total, fair value		$ 202	[4]	205
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	10 months 24 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	2 years 10 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	6 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	13 years 7 months 6 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	5 years 10 months 24 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]	1.74%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	1.08%
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]	1.09%
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]	$ 178
Available-for-sale securities, maturing after one year through five years, amortized cost	[5],[6]	1,451
Available-for-sale securities, maturing after five years through ten years, amortized cost	[5],[6]	6,624
Available-for-sale securities, maturing after ten years, amortized cost	[5],[6]	434
Available-for-sale securities, Amortized Cost		8,687	[5],[6]	8,166
Available-for-sale securities, maturing in one year or less, fair value	[5],[6]	182
Available-for-sale securities, maturing after one year through five years, fair value	[5],[6]	1,539
Available-for-sale securities, maturing after five years through ten years, fair value	[5],[6]	7,066
Available-for-sale securities, maturing after ten years, fair value	[5],[6]	428
Available-for-sale securities, total, fair value		$ 9,215	[5],[6]	8,861
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[5],[6]	8 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[5],[6]	3 years 6 months
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]	6 years 8 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[5],[6]	4.21%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[5],[6]	4.39%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[5],[6]	3.86%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[5],[6]	2.57%
Available-for-sale securities, total, weighted-average yield	[3],[5],[6]	3.89%
Other [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 7
Available-for-sale securities, Amortized Cost		7		9
Available-for-sale securities, maturing in one year or less, fair value		7
Available-for-sale securities, total, fair value		$ 7		$ 9
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	[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.