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Stock-Based Compensation - Expense (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Selling, general and administrative expense and operating expenses (excluding depreciation and amortization)        
Stock-Based Compensation        
Non-cash compensation expense $ 4.5 $ 2.6 $ 11.6 $ 8.3