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Property, Plant and Equipment - Narrative (Details) - Midstream Generator Assets - USD ($)
3 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Property, Plant and Equipment    
Assets held-for-sale   $ 5,700,000
Net realizable value of assets held for sale   1,600,000
Impairment of assets to be disposed of   $ 4,100,000
Gain (loss) on sale of assets and asset impairment charges $ 0