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Basis of presentation and significant accounting policies - Property and equipment (Details)
3 Months Ended
Mar. 31, 2017
USD ($)
$ / Boe
Mar. 31, 2016
USD ($)
$ / Boe
Dec. 31, 2016
USD ($)
Property, Plant and Equipment, Net, by Type [Abstract]      
Evaluated oil and natural gas properties $ 5,560,767,000   $ 5,488,756,000
Less accumulated depletion and impairment (4,544,547,000)   (4,514,183,000)
Evaluated oil and natural gas properties, net 1,016,220,000   974,573,000
Unevaluated properties not being depleted 215,598,000   221,281,000
Property and equipment, net $ 1,401,484,000   1,366,867,000
Depletion expense (in dollars per BOE) | $ / Boe 6.45 9.00  
Capitalized employee-related costs $ 5,210,000 $ 3,196,000  
Discount rate full cost ceiling (percent) 10.00%    
Midstream Service Assets      
Property, Plant and Equipment, Net, by Type [Abstract]      
Midstream service assets $ 151,819,000   150,629,000
Less accumulated depreciation and amortization (26,532,000)   (24,389,000)
Property and equipment, net 125,287,000   126,240,000
Other fixed assets      
Property, Plant and Equipment, Net, by Type [Abstract]      
Other fixed assets 52,695,000   52,491,000
Less accumulated depreciation and amortization (23,230,000)   (22,632,000)
Property and equipment, net 29,465,000   29,859,000
Land      
Property, Plant and Equipment, Net, by Type [Abstract]      
Other fixed assets 14,914,000   $ 14,914,000
Level 3 | Fair Value, Measurements, Nonrecurring [Member]      
Property, Plant and Equipment, Net, by Type [Abstract]      
Unamortized costs capitalized before related deferred income taxes exceed ceiling limitations, expense $ 0 $ 161,100,000