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Cash and Marketable Securities - Components of Investments (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Cash and Cash Equivalents [Line Items]        
Cash & Cash Equivalents $ 54.4 $ 40.5 $ 61.6 $ 66.7
Total restricted cash 62.4 60.9    
Restricted Marketable Securities 62.0 57.7    
Total 178.8 159.1    
Cash [Member]        
Cash and Cash Equivalents [Line Items]        
Cash & Cash Equivalents 54.4 40.5    
Total restricted cash 62.4 60.9    
Restricted Marketable Securities 0.0 0.0    
Total 116.8 101.4    
Equity Securities [Member]        
Cash and Cash Equivalents [Line Items]        
Cash & Cash Equivalents 0.0 0.0    
Total restricted cash 0.0 0.0    
Restricted Marketable Securities 62.0 57.7    
Total $ 62.0 $ 57.7