Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Narrative (Details)

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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
MMcfe
well
$ / bbl
$ / MMBTU
Dec. 31, 2016
MMcfe
well
$ / bbl
$ / MMBTU
Dec. 31, 2015
MMcfe
$ / bbl
$ / MMBTU
Reserve Quantities [Line Items]      
Future development costs estimated to be spent during 2018 | $ $ 551.0    
Future development costs estimated to be spent during 2019 | $ 458.8    
Future development costs estimated to be spent during 2020 | $ $ 514.5    
Oil derivatives      
Reserve Quantities [Line Items]      
Price per unit (usd per barrel or MMbtu) | $ / bbl 51.34 42.75 50.28
Natural gas, per MMbtu      
Reserve Quantities [Line Items]      
Price per unit (usd per barrel or MMbtu) | $ / MMBTU 2.98 2.48 2.59
Natural gas liquids      
Reserve Quantities [Line Items]      
Price per unit (usd per barrel or MMbtu) | $ / bbl 18.40 9.91 13.21
SCOOP Properties      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, purchase of mineral in place 1,500,000    
Utica Shale      
Reserve Quantities [Line Items]      
Increase (decrease) in reserve during the period 1,600,000 1,100,000 1,044,500
Planned unit development | well 10 34  
Proved developed and undeveloped reserve, production increase, percentage   14.50%  
Utica Shale | Improved performance      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate (201,300) (26,700)  
Utica Shale | Higher commodity prices      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate (214,100)    
Utica Shale | Change in ownership interest      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate (95,900)    
Utica Shale | Decline in performance      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate (133,000)    
Utica Shale | Rescheduled drilling      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate (45,700) 17,400  
Planned unit development | well   4  
Proved undeveloped reserves, postponement of drilling period   5 years  
Utica Shale | Lower commodity prices      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate   227,900  
Planned unit development loss | well   35  
Development wells drilled | well   67  
Utica Shale | Exclusion of PUD locations      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate     444,314
Utica Shale | Gulfport | Rescheduled drilling      
Reserve Quantities [Line Items]      
Planned unit development | well 5    
Utica Shale | Third Party | Rescheduled drilling      
Reserve Quantities [Line Items]      
Planned unit development | well 5    
Louisiana Field | Rescheduled drilling      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, revision of previous estimate (600)    
Planned unit development | well   2  
Proved undeveloped reserves, postponement of drilling period   5 years  
Paloma and AEU      
Reserve Quantities [Line Items]      
Proved developed and undeveloped reserve, purchase of mineral in place     371,663
Investment in Grizzly Oil Sands ULC      
Reserve Quantities [Line Items]      
Equity investment ownership interest 24.9999%