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Derivatives Sales, Receivables and Postretirement Cost Sharing Asset by Major Customer (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Revenue, Major Customer [Line Items]      
Total GM postretirement cost sharing asset $ 265.5 $ 249.0  
General Motors [Member]      
Revenue, Major Customer [Line Items]      
Entity-wide revenue, major customer, percentage 47.00% 67.00% 66.00%
Fair value, concentration of risk, accounts receivable $ 466.8 $ 369.1  
FCA [Member]      
Revenue, Major Customer [Line Items]      
Entity-wide revenue, major customer, percentage 14.00% 18.00% 20.00%
Fair value, concentration of risk, accounts receivable $ 129.0 $ 87.3