Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation - Outstanding Awards and Exercises (Details)

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Stock-Based Compensation - Outstanding Awards and Exercises (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Series A common stock      
Options additional disclosures      
Shares reserved for future issuance upon exercise of stock options 4,000    
Series C common stock      
Options additional disclosures      
Shares reserved for future issuance upon exercise of stock options 1,900,000    
Awards      
Compensation cost not yet recognized      
Unrecognized compensation cost options $ 9.7    
Period over which unrecognized compensation cost will be recognized 2 years 6 months    
Awards | Series A common stock      
Options      
Outstanding beginning balance (in shares) 393,000    
Exercised (in shares) (389,000)    
Outstanding ending balance (in shares) 4,000 393,000  
Number of awards exercisable (in shares) 4,000    
WAEP      
WAEP Outstanding beginning balance (in dollars per share) $ 33.31    
WAEP options exercised (in dollars per share) 33.16    
WAEP Outstanding ending balance (in dollars per share) 47.92 $ 33.31  
WAEP options exercisable (in dollars per share) $ 47.92    
Options additional disclosures      
Weighted average remaining contractual life outstanding 2 years    
Weighted average remaining contractual life exercisable 2 years    
Awards | Series C common stock      
Options      
Outstanding beginning balance (in shares) 2,356,000    
Options granted (in shares) 351,000    
Exercised (in shares) (775,000)    
Outstanding ending balance (in shares) 1,932,000 2,356,000  
Number of awards exercisable (in shares) 1,622,000    
WAEP      
WAEP Outstanding beginning balance (in dollars per share) $ 43.77    
WAEP Options granted (in dollars per share) 117.52    
WAEP options exercised (in dollars per share) 33.14    
WAEP Outstanding ending balance (in dollars per share) 61.43 $ 43.77  
WAEP options exercisable (in dollars per share) $ 50.00    
Options additional disclosures      
Weighted average remaining contractual life outstanding 5 years 3 months 18 days    
Weighted average remaining contractual life exercisable 4 years 10 months 24 days    
Aggregate intrinsic value outstanding $ 124.0    
Aggregate intrinsic value exercisable 123.0    
Options      
Options additional disclosures      
Aggregate intrinsic value of options exercised during period $ 91.7 $ 3.0 $ 8.1
Options | Series A common stock      
Options      
Options granted (in shares) 0    
Options | Series B common stock      
Options      
Outstanding ending balance (in shares) 0