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Segment Reporting - Narrative (Details)
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
segment
member
Dec. 31, 2018
USD ($)
member
Dec. 31, 2017
USD ($)
member
Segment Reporting Information [Line Items]      
Number of reportable segments | segment 2    
Revenue $ 6,547 $ 6,276 $ 5,843
Trading and Clearing Segment      
Segment Reporting Information [Line Items]      
Revenue $ 3,887 $ 3,717 $ 3,333
Revenue | Trading and Clearing Segment      
Segment Reporting Information [Line Items]      
Number of members | member 1 1 2
Revenue $ 368 $ 406 $ 477
Concentration risk, percentage 14.00% 17.00% 22.00%