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Revenue - Contract costs (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Abstract]            
Contract with Customer, Liability $ 15.7 $ 28.9 $ 15.7 $ 28.9 $ 26.5 $ 24.9
Contract with Customer, Liability, Revenue Recognized $ 14.0 $ 26.4 $ 26.5 $ 24.9