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Significant Accounting Policies Restricted Cash (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]      
Percentage of FIFO Inventory 55.00%    
Restricted Cash and Cash Equivalents, Current $ 3.8 $ 2.7  
Increase (Decrease) in Restricted Cash $ 1.2 2.5 $ (14.8)
Percentage of LIFO Inventory 45.00%    
Financial Guarantee [Member]      
Restricted Cash and Cash Equivalents Items [Line Items]      
Increase (Decrease) in Restricted Cash $ 0.5    
Dividend Declared [Member]      
Restricted Cash and Cash Equivalents Items [Line Items]      
Increase (Decrease) in Restricted Cash   $ 0.6