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Stock-based Payments - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Dec. 31, 2014
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Pre-IPO stock-based compensation   $ 10.8 $ 6.2 $ 25.7 $ 20.5  
Time-Based Options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Pre-IPO stock-based compensation   4.5 $ 0.5 6.8 $ 1.7  
Unrecognized compensation cost related to non-vested time-based option awards   4.1   4.1    
Performance-Based Options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Unrecognized compensation cost related to non-vested time-based option awards   2.2   2.2    
Cassidy Turley, Inc.            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Payment made to acquire business $ 179.8          
Period compensation expense will be recognized 4 years          
Accrued value of cash-settled portion, deferred purchase obligation   $ 122.6   $ 122.6   $ 105.6