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Property, Plant, and Equipment (Details Textual) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]    
Property, plant, and equipment, net $ 5,966.8 $ 5,348.0
Nonoperating plants    
Property, Plant and Equipment [Line Items]    
Property, plant, and equipment, net 65.1  
Non-Leasing Group    
Property, Plant and Equipment [Line Items]    
2017 10.0  
2018 6.8  
2019 4.1  
2020 1.6  
2021 0.8  
Thereafter $ 3.7