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, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes In Standardized Measure of Discontinued Future Net Cash Flows [Line Items]      
Sales and transfers of oil and gas produced, net of production costs $ (756,257) $ (312,291) $ (486,185)
Net changes in prices, production costs, and development costs 913,714 (146,518) (1,412,181)
Acquisition of oil and gas reserves in place 703,866 0 83,340
Extensions and discoveries 618,039 186,909 262,895
Previously estimated development costs incurred during the period 390,673 176,218 117,540
Revisions of previous quantity estimates, less related production costs 155,200 (38,448) (98,162)
Accretion of discount 68,804 76,433 142,717
Net changes in income taxes (231,545) (6,495) 412,240
Change in production rates and other 93,030 (12,099) 314,960
Total change in standardized measure of discounted future net cash flows 1,955,524 (76,291) (662,836)
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 0 114
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 47
Revisions of previous quantity estimates, less related production costs 0 0 (103,282)
Accretion of discount 0 0 9,375
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 $ (93,746)