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Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
£ / $
Dec. 31, 2020
USD ($)
€ / $
Dec. 31, 2019
USD ($)
£ / $
Dec. 31, 2019
USD ($)
€ / $
Cash and Cash Equivalents [Line Items]          
Restricted cash $ 0.0 $ 0.0 $ 0.0 $ 0.1 $ 0.1
Average foreign currency exchange rate, translation   0.7805 0.8888 0.7820 0.8943
Increase of average foreign currency exchange rate, translation   0.05 0.05    
GBP          
Cash and Cash Equivalents [Line Items]          
Effect of exchange rate on revenue during the year 0.3        
Effect of exchange rate on operating income during the year 0.2        
Effect of exchange on revenue per 0.05 increase in foreign exchange rate 8.5        
Effect of exchange on operating income per 0.05 increase in foreign exchange rate 0.7        
Euros          
Cash and Cash Equivalents [Line Items]          
Effect of exchange rate on revenue during the year 0.1        
Effect of exchange rate on operating income during the year 0.3        
Effect of exchange on revenue per 0.05 increase in foreign exchange rate 0.1        
Effect of exchange on operating income per 0.05 increase in foreign exchange rate $ 0.3