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Cash and Marketable Securities - Components of Investments (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and Cash Equivalents [Line Items]        
Cash & Cash Equivalents $ 69.2 $ 54.4 $ 40.5 $ 61.6
Restricted Cash 64.3 62.4    
Restricted marketable securities 62.0 62.0    
Total 195.5 178.8    
Cash [Member]        
Cash and Cash Equivalents [Line Items]        
Cash & Cash Equivalents 69.2 54.4    
Restricted Cash 64.3 62.4    
Restricted marketable securities 0.0 0.0    
Total 133.5 116.8    
Equity Securities [Member]        
Cash and Cash Equivalents [Line Items]        
Cash & Cash Equivalents 0.0 0.0    
Restricted Cash 0.0 0.0    
Restricted marketable securities 62.0 62.0    
Total $ 62.0 $ 62.0