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Cash and Marketable Securities - Investing Information Related to Marketable Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investment information related to restricted marketable securities      
Gross realized gains $ 0.0 $ 0.0 $ 1.2
Gross realized losses 0.0 0.0 0.0
Restricted Marketable Securities [Member]      
Investment information related to restricted marketable securities      
Proceeds from sales of available-for-sale securities 4.0 0.0 0.0
Nonrestricted Marketable Securities [Member]      
Investment information related to restricted marketable securities      
Proceeds from sales of available-for-sale securities $ 0.0 $ 0.0 $ 12.8