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Fair Value Measurements (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
agreement
Dec. 31, 2016
USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying value of other investments that lack a readily available market $ 9.6 $ 16.1
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 1,838.7 1,714.0
Short-term borrowings & notes payable 249.5 194.4
Cash & cash equivalents 16.6 68.6
Natural gas purchase instrument assets 0.5 0.0
Natural gas purchase instrument liabilities 4.5 0.0
Interest rate swap liabilities 1.4 0.0
Restricted cash 0.0 0.9
Est. Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 1,981.2 1,835.8
Short-term borrowings & notes payable 249.5 194.4
Cash & cash equivalents 16.6 68.6
Natural gas purchase instrument assets 0.5 0.0
Natural gas purchase instrument liabilities 4.5 0.0
Interest rate swap liabilities 1.4 0.0
Restricted cash $ 0.0 $ 0.9
Forward Contracts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Number of purchase agreements | agreement 4  
Forward purchase arrangements 5 years