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Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2016 | Feb. 01, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Asset retirement obligation capitalized | $ 42,270 | $ 10,971 | $ 8,800 |  |
Asset retirement obligation, revised |  | $ 0 |  |  |
Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Asset retirement obligation capitalized | 16,300 |  |  |  |
Asset retirement obligation, revised | $ 25,970 |  |  |  |
Level 3 | Investment in Grizzly Oil Sands ULC |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Fair value of equity investment |  |  |  | $ 39,100 |
Level 3 | Strike Force Midstream LLC |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Fair value of equity investment |  |  |  |  | $ 22,500

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