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Revenue - Summary of Annual Service Fees and Regulatory Compliance Fees (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Disaggregation Of Revenue [Line Items]        
Gross revenue $ 141.8 $ 180.5 $ 341.2 $ 335.5
Annual Service Fees and Regulatory Compliance Fees        
Disaggregation Of Revenue [Line Items]        
Gross revenue 3.4 2.9 6.8 5.8
Amount of These Fees Included in Deferred Revenue at Beginning of Period        
Disaggregation Of Revenue [Line Items]        
Gross revenue $ 1.7 $ 1.4 $ 2.8 $ 2.7