Debt - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Line of Credit Facility [Line Items]
Other short-term debt	$ 5,000,000		$ 0
Proceeds from commercial paper	347,000,000	$ 812,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	3,400,000,000
Additional borrowing capacity (up to)	975,000,000
Amount of debt outstanding	0
Amount required to stop broker-dealer subsidiary commitments	105,000,000
Funds available to use for working capital and general and corporate purposes	2,000,000,000.0
Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	20,000,000
Other short-term debt	5,000,000
Commercial Paper | Revolving Credit Facility
Line of Credit Facility [Line Items]
Funds reserved for commercial paper program	$ 1,300,000,000
Weighted average interest rate	2.56%
Weighted average maturity period	26 days
Commercial Paper | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Commercial paper maturities	1 day
Commercial Paper | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Commercial paper maturities	71 days
BondPoint and TMC Bonds
Line of Credit Facility [Line Items]
Proceeds from commercial paper	$ 347,000,000