Investment Securities - Amortized Cost, Fair Value and Yield by Maturity Date (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Jun. 30, 2020		Dec. 31, 2019
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 124,513	[1]	$ 122,117
Available-for-sale securities, total, fair value	[2]	$ 128,120	[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		2.05%	[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		$ 5,453
Available-for-sale securities, maturing after one year through five years, amortized cost		9,834
Available-for-sale securities, maturing after five years through ten years, amortized cost		4,044
Available-for-sale securities, maturing after ten years, amortized cost		66
Available-for-sale securities, Amortized Cost		19,397		$ 19,845
Available-for-sale securities, maturing in one year or less, fair value		5,486
Available-for-sale securities, maturing after one year through five years, fair value		10,167
Available-for-sale securities, maturing after five years through ten years, fair value		4,259
Available-for-sale securities, maturing after ten years, fair value		66
Available-for-sale securities, total, fair value		$ 19,978		19,839
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 2 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 7 months 6 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		10 years 9 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 8 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.64%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3]	1.69%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3]	1.51%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3]	1.78%
Available-for-sale securities, total, weighted-average yield	[3]	1.64%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost	[4]	$ 1,194
Available-for-sale securities, maturing after one year through five years, amortized cost	[4]	91,418
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	4,601
Available-for-sale securities, maturing after ten years, amortized cost	[4]	59
Available-for-sale securities, Amortized Cost	[4]	97,272
Available-for-sale securities, maturing in one year or less, fair value	[4]	1,215
Available-for-sale securities, maturing after one year through five years, fair value	[4]	93,694
Available-for-sale securities, maturing after five years through ten years, fair value	[4]	4,797
Available-for-sale securities, maturing after ten years, fair value	[4]	59
Available-for-sale securities, total, fair value	[4]	$ 99,765
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[4]	9 months 18 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[4]	2 years 7 months 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	7 years 10 months 24 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	12 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	2 years 9 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[4]	2.33%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[4]	1.98%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	1.56%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[4]	1.22%
Available-for-sale securities, total, weighted-average yield	[3],[4]	1.97%
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]	$ 360
Available-for-sale securities, maturing after five years through ten years, amortized cost	[4]	1
Available-for-sale securities, Amortized Cost	[4]	361
Available-for-sale securities, maturing after one year through five years, fair value	[4]	366
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]	$ 368
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 10 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years	[4]	5 years 8 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years	[4]	14 years 7 months 6 days
Available-for-sale securities, total, weighted-average maturity in years	[4]	2 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]	3.59%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[4]	1.00%
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]	3.58%
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]	$ 67
Available-for-sale securities, maturing after one year through five years, amortized cost	[5],[6]	863
Available-for-sale securities, maturing after five years through ten years, amortized cost	[5],[6]	6,263
Available-for-sale securities, maturing after ten years, amortized cost	[5],[6]	277
Available-for-sale securities, Amortized Cost		7,470	[5],[6]	6,499
Available-for-sale securities, maturing in one year or less, fair value	[5],[6]	68
Available-for-sale securities, maturing after one year through five years, fair value	[5],[6]	904
Available-for-sale securities, maturing after five years through ten years, fair value	[5],[6]	6,736
Available-for-sale securities, maturing after ten years, fair value	[5],[6]	288
Available-for-sale securities, total, fair value		$ 7,996	[5],[6]	$ 6,814
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years	[5],[6]	4 months 24 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years	[5],[6]	2 years 8 months 12 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]	10 years 2 months 12 days
Available-for-sale securities, total, weighted-average maturity in years	[5],[6]	6 years 6 months
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3],[5],[6]	5.64%
Available-for-sale securities, maturing after one year through five years, weighted-average yield	[3],[5],[6]	4.37%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield	[3],[5],[6]	4.15%
Available-for-sale securities, maturing after ten years, weighted-average yield	[3],[5],[6]	3.16%
Available-for-sale securities, total, weighted-average yield	[3],[5],[6]	4.15%
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		7 months 6 days
Available-for-sale securities, total, weighted-average maturity in years		7 months 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield	[3]	1.65%
Available-for-sale securities, total, weighted-average yield	[3]	1.65%

[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.