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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
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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)

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- Definition Net change in sales and transfer prices and in production (lifting) costs related to future production and in the evaluation of the costs that will be required in future periods to make it possible to extract or process proved oil and gas reserves. + References No definition available. + Details
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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of extensions, discoveries and improved recovery of proved oil and gas reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62500-109447 + Details
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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of change in the estimated future income tax on future pretax net cash flows related to the entity's proved oil and gas reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62500-109447 + Details
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