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, 2018
Dec. 31, 2017
Dec. 31, 2016
Changes In Standardized Measure of Discontinued Future Net Cash Flows [Line Items]      
Sales and transfers of oil and gas produced, net of production costs $ (1,063,215) $ (756,257) $ (312,291)
Net changes in prices, production costs, and development costs 590,519 913,714 (146,518)
Acquisition of oil and gas reserves in place 0 703,866 0
Extensions and discoveries 519,137 618,039 186,909
Previously estimated development costs incurred during the period 402,156 390,673 176,218
Revisions of previous quantity estimates, less related production costs (356,933) 155,200 (38,448)
Sales of oil and gas reserves in place (25,882) 0 0
Accretion of discount 264,356 68,804 76,433
Net changes in income taxes (185,157) (231,545) (6,495)
Change in production rates and other 194,180 93,030 (12,099)
Total change in standardized measure of discounted future net cash flows 339,161 1,955,524 (76,291)
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