Investment Securities - Amortized Cost, Fair Value and Yield by Maturity Date (Detail) - USD ($) $ in Millions		6 Months Ended		12 Months Ended
		Jun. 30, 2021		Dec. 31, 2020
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 159,308	[1]	$ 133,611
Available-for-sale securities, total, fair value	[2]	$ 160,288	[1]	$ 136,840
Available-for-sale securities, total, weighted-average maturity in years		5 years 7 months 6 days	[1]	3 years 4 months 24 days
Available-for-sale securities, total, weighted-average yield		1.65%	[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		$ 3,478
Available-for-sale securities, maturing after one year through five years, amortized cost		10,441
Available-for-sale securities, maturing after five years through ten years, amortized cost		7,756
Available-for-sale securities, maturing after ten years, amortized cost		928
Available-for-sale securities, Amortized Cost		22,603		$ 21,954
Available-for-sale securities, maturing in one year or less, fair value		3,495
Available-for-sale securities, maturing after one year through five years, fair value		10,587
Available-for-sale securities, maturing after five years through ten years, fair value		7,670
Available-for-sale securities, maturing after ten years, fair value		903
Available-for-sale securities, total, fair value		$ 22,655		22,391
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		3 months 18 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		2 years 8 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 9 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		12 years 8 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		4 years 6 months
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.70%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	1.28%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3]	1.25%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3]	1.66%
Available-for-sale securities, total, weighted-average yield	[3]	1.35%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost	[4]	$ 99
Available-for-sale securities, maturing after one year through five years, amortized cost	[4]	48,293
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	78,566
Available-for-sale securities, maturing after ten years, amortized cost	[4]	47
Available-for-sale securities, Amortized Cost	[4]	127,005
Available-for-sale securities, maturing in one year or less, fair value	[4]	96
Available-for-sale securities, maturing after one year through five years, fair value	[4]	49,334
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	77,805
Available-for-sale securities, maturing after ten years, fair value	[4]	48
Available-for-sale securities, total, fair value	[4]	$ 127,283
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[5]	7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	3 years 6 months
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	7 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	11 years 8 months 12 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	5 years 8 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[4]	1.93%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[4]	1.45%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	1.60%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[4]	1.11%
Available-for-sale securities, total, weighted-average yield	[3],[4]	1.54%
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]	191
Available-for-sale securities, Amortized Cost		194	[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]	195
Available-for-sale securities, maturing after ten years, fair value	[4]	1
Available-for-sale securities, total, fair value		$ 200	[4]	205
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[5]	9 months 18 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	2 years 8 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	5 years 9 months 18 days
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 8 months 12 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]	1.69%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[4]	0.89%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[4]	2.41%
Available-for-sale securities, total, weighted-average yield	[4]	0.90%
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	[6],[7]	$ 231
Available-for-sale securities, maturing after one year through five years, amortized cost	[6],[7]	1,878
Available-for-sale securities, maturing after five years through ten years, amortized cost	[6],[7]	6,963
Available-for-sale securities, maturing after ten years, amortized cost	[6],[7]	427
Available-for-sale securities, Amortized Cost		9,499	[6],[7]	8,166
Available-for-sale securities, maturing in one year or less, fair value	[6],[7]	235
Available-for-sale securities, maturing after one year through five years, fair value	[6],[7]	2,014
Available-for-sale securities, maturing after five years through ten years, fair value	[6],[7]	7,464
Available-for-sale securities, maturing after ten years, fair value	[6],[7]	430
Available-for-sale securities, total, fair value		$ 10,143	[6],[7]	8,861
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[6],[7]	7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[6],[7]	3 years 8 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[6],[7]	7 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[6],[7]	13 years 10 months 24 days
Available-for-sale securities, total, weighted-average maturity in years	[6],[7]	6 years 6 months
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[6],[7]	4.25%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[6],[7]	4.34%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[6],[7]	3.66%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[6],[7]	2.57%
Available-for-sale securities, total, weighted-average yield	[3],[6],[7]	3.76%
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 10 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 10 months 24 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 asset and mortgage-backed securities included above is presented based upon weighted-average maturities that take into account anticipated future prepayments.

[6] 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.

[7] 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.