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Commitments and Contingencies - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Loss Contingencies [Line Items]        
Contribution to noncontrolling interest     $ 116 $ 69
Affiliated Entity        
Loss Contingencies [Line Items]        
Contribution to noncontrolling interest     20  
CME Group | S&P DJ Indices        
Loss Contingencies [Line Items]        
Revenues earned under license agreement $ 28 $ 18 $ 82 $ 56
Noncontrolling interest ownership by noncontrolling owners (as a percent) 27.00%   27.00%