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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, 2018 | Dec. 31, 2017
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Reserve Quantities [Line Items] |  |
Proven properties | $ 7,153,799 | $ 6,256,182
Unproven properties | 2,873,037 | 2,912,974
Capitalized costs, gross | 10,026,836 | 9,169,156
Accumulated depreciation, depletion, amortization and impairment reserve | (4,613,293) | (4,136,777)
Net capitalized costs | 5,413,543 | 5,032,379
Investment in Grizzly Oil Sands ULC |  |
Reserve Quantities [Line Items] |  |
Proven properties | 67,475 | 73,818
Unproven properties | 79,605 | 86,540
Capitalized costs, gross | 147,080 | 160,358
Accumulated depreciation, depletion, amortization and impairment reserve | (1,553) | (1,693)
Net capitalized costs | $ 145,527 | $ 158,665

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- Definition Amount of accumulated depreciation, depletion, amortization and valuation allowance relating to oil and gas producing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447 + Details
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- Definition The sum of capitalized costs relating to oil and gas producing activities before accounting for accumulated depreciation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447 + Details
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- Definition Net amount of capitalized costs relating to oil and gas producing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447 + Details
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- Definition Capitalized costs of proved properties incurred for any combination mineral interests acquisitions; wells and related equipment; support equipment and facilities; and uncompleted wells and equipment and other costs not previously disclosed within this table. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -URI http://asc.fasb.org/subtopic&trid=2145654 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447 + Details
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