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Stockholders' Equity - Employee Stock Purchase Plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock Compensation Plans      
Number of shares issued 258,453 239,590 233,712
Stock compensation expense $ 177.9 $ 166.6 $ 148.2
Remaining shares available 5,515,182 9,882,122  
Employee Stock      
Stock Compensation Plans      
Employment period for eligibility to participate in the plan 1 month    
Number of shares issued 258,453 239,590 233,712
Stock compensation expense $ 4.6 $ 3.4 $ 3.7
Remaining shares available 945,478    
Employee Stock | Minimum      
Stock Compensation Plans      
Number of employee working hours per week for eligibility to participate in the plan 20 hours