Debt - Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Aug. 21, 2020
Line of Credit Facility [Line Items]
Other short-term debt	$ 8,000,000	$ 8,000,000		$ 6,000,000
Repayments of commercial paper		1,081,000,000	$ (1,149,000,000)
Repayments of debt facilities		1,248,000,000	$ 1,258,000,000
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	0
Currently available for borrowing	3,800,000,000	3,800,000,000
Amount required to stop broker-dealer subsidiary commitments	172,000,000	172,000,000
Funds available to use for working capital and general and corporate purposes	2,300,000,000	2,300,000,000
Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	20,000,000	20,000,000
Other short-term debt	8,000,000	8,000,000
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 12,900,000,000	$ 12,900,000,000
Debt instrument, term (in months)		15 years
Weighted average interest rate	3.00%	3.00%
Senior Notes | 2023 Senior Notes (0.70% senior unsecured notes due June 15, 2023)
Line of Credit Facility [Line Items]
Extinguishment of debt	$ 1,250,000,000
Interest debt expense	4,000,000
Commercial Paper | Revolving Credit Facility
Line of Credit Facility [Line Items]
Funds reserved for commercial paper program	$ 1,300,000,000	$ 1,300,000,000
Repayments of commercial paper		1,100,000,000
Repayments of debt facilities		$ 1,200,000,000
Weighted average interest rate		0.24%
Weighted average maturity period (in days)		28 days
Commercial Paper | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Commercial paper maturities (in days)		1 day
Commercial Paper | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Commercial paper maturities (in days)		75 days