Quarterly report pursuant to Section 13 or 15(d)

Property and equipment - Narrative (Details)

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Property and equipment - Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Property, Plant and Equipment [Line Items]          
Depreciation, depletion, amortization and accretion expense $ 47,100,000 $ 8,200,000 $ 103,800,000 $ 25,700,000  
Depreciation and amortization expense 47,469,000 8,383,000 104,784,000 26,150,000  
Impairment charges related to proved and unproved oil and natural gas properties 0 0 0 0  
Capitalized costs of unproved properties excluded from amortization     46,200,000   $ 57,200,000
Other property and equipment          
Property, Plant and Equipment [Line Items]          
Depreciation and amortization expense $ 200,000 $ 100,000 $ 500,000 $ 300,000