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Commitments and Contingencies - Narrative (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Aug. 30, 2015
shareholder
Loss Contingencies [Line Items]        
Noncontrolling interest ownership by noncontrolling owners (as a percent) 27.00%      
Shareholder Derivative Action        
Loss Contingencies [Line Items]        
Number of shareholders commencing punitive derivative action | shareholder       2
CME Group | S&P/DJ Indices        
Loss Contingencies [Line Items]        
Revenues earned under license agreement | $ $ 74 $ 76 $ 63  
Noncontrolling interest ownership by noncontrolling owners (as a percent) 27.00%