Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Costs Incurred In Oil and Gas Property Acquisition and Development Activities (Details)

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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Costs Incurred In Oil and Gas Property Acquisition and Development Activities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Acquisition $ 1,951,281 $ 152,887 $ 810,755
Development of proved undeveloped properties 994,237 423,998 642,811
Exploratory 0 0 0
Recompletions 14,289 16,386 13,894
Capitalized asset retirement obligation 42,270 10,971 8,800
Total 3,002,077 604,242 1,476,260
Investment in Grizzly Oil Sands ULC      
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Acquisition 503 357 396
Development of proved undeveloped properties 0 0 47
Exploratory 0 0 0
Capitalized asset retirement obligation (524) 784 282
Total $ (21) $ 1,141 $ 725