AFS and HTM Debt Securities, Investment Grade Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available For Sale And Held To Maturity Debt Securities Credit Ratings [Abstract]
Available-for-sale, fair value	$ 251,229	$ 263,459	[1]
Available-for-sale, percent of investment grade	99.00%	99.00%
Debt Securities, Held-to-maturity	$ 169,920	$ 153,933
Held-to-maturity, percent of investment grade	99.00%	99.00%
Securities Of US Treasury and federal agencies [Member]
Available For Sale And Held To Maturity Debt Securities Credit Ratings [Abstract]
Available-for-sale, fair value	$ 171,250	$ 177,413
Available-for-sale, percent of investment grade	100.00%	100.00%
Debt Securities, Held-to-maturity	$ 154,734	$ 139,619
Held-to-maturity, percent of investment grade	100.00%	100.00%
Securities of U.S. states and political subdivisions [Member]
Available For Sale And Held To Maturity Debt Securities Credit Ratings [Abstract]
Available-for-sale, fair value	$ 38,144	$ 40,337
Available-for-sale, percent of investment grade	99.00%	99.00%
Debt Securities, Held-to-maturity	$ 14,313	$ 13,486
Held-to-maturity, percent of investment grade	100.00%	100.00%
Collateralized loan obligations [Member]
Available For Sale And Held To Maturity Debt Securities Credit Ratings [Abstract]
Available-for-sale, fair value	$ 24,582	$ 29,055
Available-for-sale, percent of investment grade	100.00%	100.00%
All other debt securities [Member]
Available For Sale And Held To Maturity Debt Securities Credit Ratings [Abstract]
Available-for-sale, fair value	$ 17,253	$ 16,654
Available-for-sale, percent of investment grade	82.00%	82.00%
Debt Securities, Held-to-maturity	$ 873	$ 828
Held-to-maturity, percent of investment grade	7.00%	4.00%
Internal Investment Grade [Member]
Available For Sale And Held To Maturity Debt Securities Credit Ratings [Abstract]
Available-for-sale, fair value	$ 1,200	$ 2,200

[1] (1) Prior to our adoption of CECL on January 1, 2020, the allowance for credit losses (ACL) related to available-for-sale (AFS) and held-to-maturity (HTM) debt securities was not applicable and is therefore presented as $0 at December 31, 2019. For more information, see Note 1 (Summary of Significant Accounting Policies) to Financial Statements in this Report.