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Acquired Intangible Assets, Amortization Expense (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Amortization of acquired intangible assets [Abstract]        
Amortization expense $ 2.4 $ 2.0 $ 6.5 $ 6.1
Estimated amortization expense, remainder of 2021 2.2   2.2  
Estimated amortization expense in year 2022 8.6   8.6  
Estimated amortization expense in year 2023 8.3   8.3  
Estimated amortization expense in year 2024 8.2   8.2  
Estimated amortization expense in years 2025 through 2041 $ 78.0   $ 78.0