Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details)

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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Changes In Standardized Measure of Discontinued Future Net Cash Flows [Line Items]      
Sales and transfers of oil and gas produced, net of production costs $ (734,354) $ (1,063,215) $ (756,257)
Net changes in prices, production costs, and development costs (1,372,443) 590,519 913,714
Acquisition of oil and gas reserves in place 0 0 703,866
Extensions and discoveries 388,151 519,137 618,039
Previously estimated development costs incurred during the period 405,979 402,156 390,673
Revisions of previous quantity estimates, less related production costs (321,397) (356,933) 155,200
Sales of oil and gas reserves in place (48,547) (25,882) 0
Accretion of discount 298,273 264,356 68,804
Net changes in income taxes 424,628 (185,157) (231,545)
Change in production rates and other (319,428) 194,180 93,030
Total change in standardized measure of discounted future net cash flows (1,279,138) 339,161 1,955,524
Investment in Grizzly Oil Sands ULC      
Changes In Standardized Measure of Discontinued Future Net Cash Flows [Line Items]      
Sales and transfers of oil and gas produced, net of production costs 0 0 0
Net changes in prices, production costs, and development costs 0 0 0
Acquisition of oil and gas reserves in place 0 0 0
Extensions and discoveries 0 0 0
Previously estimated development costs incurred during the period 0 0 0
Revisions of previous quantity estimates, less related production costs 0 0 0
Accretion of discount 0 0 0
Net changes in income taxes 0 0 0
Change in production rates and other 0 0 0
Total change in standardized measure of discounted future net cash flows $ 0 $ 0 $ 0