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Other Long-term Liabilities - Changes in Deferred Revenue (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Other Liabilities Noncurrent [Abstract]      
Balance at beginning of period $ 172.0 $ 175.5  
Additions 0.3 0.4  
Revenue recognized (3.8) (3.9) $ (3.9)
Balance at end of period $ 168.5 $ 172.0 $ 175.5