Background and Basis of Presentation (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) segment		Jun. 30, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)		Sep. 30, 2019 USD ($)	[3]	Jun. 30, 2019 USD ($)	[3]	Dec. 31, 2018 USD ($)	[3]
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of reportable segments (in number of segments) | segment	2
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 8,226	[1],[2]	$ 8,184	$ 5,844	$ 3,383	[2],[3]	$ 3,299		$ 3,202		$ 3,041
Other assets	3,453				2,459
Retained earnings	$ 19,292				$ 21,290
Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				2,500
Other assets				600
Retained earnings				$ (1,900)

[1] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[2] Prior to adoption of Accounting Standards Update ("ASU") No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[3] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.