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Cash and Marketable Securities - Textual (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Cash and Cash Equivalents [Abstract]      
Restricted marketable securities $ 72.6 $ 76.1  
Unrealized net gain (loss) on marketable securities $ 0.4 $ 1.2 $ (1.7)