Debt - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Aug. 21, 2020
Line of Credit Facility [Line Items]
Other short-term debt	$ 10,000,000		$ 6,000,000
Repayments of commercial paper	2,097,000,000	$ 611,000,000
Repayments of debt facilities	0	$ 1,256,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 3,800,000,000
Additional borrowing capacity				$ 625,000,000
Amount of debt outstanding	0
Currently available for borrowing	3,800,000,000
Amount required to stop broker-dealer subsidiary commitments	172,000,000
Funds available to use for working capital and general and corporate purposes	3,300,000,000
Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	20,000,000
Other short-term debt	10,000,000
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 12,900,000,000
Debt instrument, term (in months)	15 years
Weighted average interest rate	3.00%
Other short-term debt | Revolving Credit Facility
Line of Credit Facility [Line Items]
Funds reserved for commercial paper program	$ 308,000,000
Repayments of commercial paper	2,100,000,000
Repayments of debt facilities	$ 1,200,000,000
Weighted average interest rate	0.34%
Weighted average maturity period (in days)	38 days
Other short-term debt | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Commercial paper maturities (in days)	1 day
Other short-term debt | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Commercial paper maturities (in days)	85 days