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Cash and Marketable Securities - Investing Information Related to Marketable Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investment information related to restricted marketable securities      
Proceeds from sales and maturities of available-for-sale marketable securities $ 12.6 $ 6.4 $ 0.0