Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Capitalized Costs Related to Oil and Gas Producing Activities (Details)

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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Capitalized Costs Related to Oil and Gas Producing Activities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Reserve Quantities [Line Items]    
Proven properties $ 6,256,182 $ 4,491,615
Unproven properties 2,912,974 1,580,305
Capitalized costs, gross 9,169,156 6,071,920
Accumulated depreciation, depletion, amortization and impairment reserve (4,136,777) (3,778,043)
Net capitalized costs 5,032,379 2,293,877
Investment in Grizzly Oil Sands ULC    
Reserve Quantities [Line Items]    
Proven properties 73,818 70,266
Unproven properties 86,540 80,892
Capitalized costs, gross 160,358 151,158
Accumulated depreciation, depletion, amortization and impairment reserve (1,693) (1,578)
Net capitalized costs $ 158,665 $ 149,580