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Property, Plant and Equipment - Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended
Jun. 30, 2018
Sep. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment        
Assets held-for-sale       $ 10,600,000
Property Located in Oklahoma City, OK        
Property, Plant and Equipment        
Assets held-for-sale       $ 9,300,000
Proceeds from Sale of Property Held-for-sale $ 10,400,000      
Gain (Loss) on Sale of Assets and Asset Impairment Charges   $ 1,100,000    
Midstream Generator Assets        
Property, Plant and Equipment        
Assets held-for-sale     $ 5,700,000  
Gain (Loss) on Sale of Assets and Asset Impairment Charges $ 0      
Net realizable value of assets held for sale     $ 1,600,000  
Impairment of assets to be disposed of   $ 4,100,000