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, 2019
Dec. 31, 2018
Reserve Quantities [Line Items]    
Proved properties $ 8,909,069 $ 7,153,799
Unproved properties 1,686,666 2,873,037
Capitalized costs, gross 10,595,735 10,026,836
Accumulated depreciation, depletion, amortization and impairment (7,191,957) (4,613,293)
Net capitalized costs 3,403,778 5,413,543
Investment in Grizzly Oil Sands ULC    
Reserve Quantities [Line Items]    
Proved properties 64,476 67,475
Unproved properties 85,395 79,605
Capitalized costs, gross 149,871 147,080
Accumulated depreciation, depletion, amortization and impairment (1,634) (1,553)
Net capitalized costs $ 148,237 $ 145,527