Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Results of Operations for Producing Activities (Details)

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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Results of Operations for Producing Activities (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
usd_per_mcf
Dec. 31, 2016
USD ($)
usd_per_mcf
Dec. 31, 2015
USD ($)
usd_per_mcf
Results of Operations for Oil and Gas Producing Activities [Line Items]      
Revenues $ 1,320,303,000 $ 385,910,000 $ 708,990,000
Production costs (350,367,000) (248,125,000) (222,805,000)
Depletion (358,792,000) (243,098,000) (335,288,000)
Impairment 0 (715,495,000) (1,440,418,000)
Results of operations, before income taxes 611,144,000 (820,808,000) (1,289,521,000)
Income Tax Expense (Benefit), Continuing Operations [Abstract]      
Current 3,362,000 0 0
Deferred (3,602,000) 0 (220,201,000)
Income tax expense (240,000) 0 (220,201,000)
Results of operations from producing activities $ 611,384,000 $ (820,808,000) $ (1,069,320,000)
Depletion per Mcf of gas equivalent (usd per Mcfe) | usd_per_mcf 0.90 0.92 1.68
Investment in Grizzly Oil Sands ULC      
Results of Operations for Oil and Gas Producing Activities [Line Items]      
Revenues $ 0 $ 0 $ 1,436,000
Production costs 0 (13,000) (1,549,000)
Depletion 0 0 (625,000)
Results of operations, before income taxes 0 (13,000) (738,000)
Income Tax Expense (Benefit), Continuing Operations [Abstract]      
Income tax expense 0 0 0
Results of operations from producing activities $ 0 $ (13,000) $ (738,000)