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Cash and Marketable Securities - Restricted Marketable Securities (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Summary of Restricted Marketable Securities    
Restricted marketable securities, cost   $ 64.0
Restricted marketable securities, gross unrealized gains   0.3
Restricted marketable securities, gross unrealized losses   (2.3)
Restricted marketable securities, fair value $ 62.0 $ 62.0