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Significant Accounting Policies Update (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Allowance for doubtful accounts $ 13   $ 13   $ 9
Trade management services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, remaining performance obligation, expected timing of satisfaction 12 months   12 months    
United States | Listing services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, remaining performance obligation, expected timing of satisfaction 12 months   12 months    
Europe | Listing services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, remaining performance obligation, expected timing of satisfaction 12 months   12 months    
Minimum | Listing services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, remaining performance obligation, expected timing of satisfaction 2 years   2 years    
Maximum | Listing services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, remaining performance obligation, expected timing of satisfaction 6 years   6 years    
Services transferred at a point in time | Market Services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue recognized (percentage) 63.00% 62.00% 64.00% 63.00%  
Services transferred over time | Market Services          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue recognized (percentage) 37.00% 38.00% 36.00% 37.00%