Significant Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended
	Sep. 30, 2020 USD ($) numberOfSegments	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | numberOfSegments	4
Cumulative-effect adjustment to beginning retained earnings	$ 6,090	$ 5,653	$ 5,309	$ 6,692	$ 8,068	$ 8,287
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Allowance for credit losses	219
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment to beginning retained earnings	$ 13,890	$ 13,384	13,022	$ 11,965	$ 12,770	12,409
Opening retained earnings adjustment: adoption of new accounting standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment to beginning retained earnings			(13)			11
Opening retained earnings adjustment: adoption of new accounting standard | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment to beginning retained earnings			(13)			$ 11
ASU 2016-13 | Opening retained earnings adjustment: adoption of new accounting standard | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment to beginning retained earnings			$ 13