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Debt - Narrative (Details) - USD ($)
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Line of Credit Facility [Line Items]      
Other short-term debt $ 11,000,000   $ 0
Proceeds from commercial paper 367,000,000 $ (35,000,000)  
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 160,000,000    
Funds available to use for working capital and general and corporate purposes 1,900,000,000    
Line of Credit      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity 20,000,000    
Other short-term debt 11,000,000    
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 2.20%    
Weighted average maturity period 27 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 77 days    
BondPoint and TMC Bonds      
Line of Credit Facility [Line Items]      
Proceeds from commercial paper $ 367,000,000