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Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Asset retirement obligation capitalized | $ 1,827 | $ 16,300 |
Asset retirement obligation, revised | 375 | $ (25,970) |
Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Asset retirement obligation capitalized | 1,800 |  |
Asset retirement obligation, revised | $ 400 |  |
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

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