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Cash and Marketable Securities - Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]    
Total restricted cash $ 62.4 $ 60.9
Affiliate cash [Member]    
Restricted Cash and Cash Equivalents Items [Line Items]    
Total restricted cash 18.1 22.9
Self-insured captive funds [Member]    
Restricted Cash and Cash Equivalents Items [Line Items]    
Total restricted cash $ 44.3 $ 38.0