Significant Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended
	Jun. 30, 2020 USD ($) numberOfSegments	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | numberOfSegments	3
Cumulative-effect adjustment to beginning retained earnings	$ 5,653	$ 5,309	$ 8,068	$ 8,287
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Allowance for credit losses	$ 225
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment to beginning retained earnings	$ 13,384	13,022	$ 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