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Cash and Marketable Securities - Restricted Marketable Securities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Summary of Restricted Marketable Securities    
Restricted marketable securities, cost $ 64.0 $ 59.6
Restricted marketable securities, gross unrealized gains 0.3 0.2
Restricted marketable securities, gross unrealized losses (2.3) (2.1)
Restricted marketable securities, fair value $ 62.0 $ 57.7