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Fair Value Measurements (Schedule of Derivative Instruments) (Details) - USD ($) $ in Thousands | 6 Months Ended |  |  |
--- | --- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016 | Mar. 31, 2016 | Feb. 29, 2016
Liabilities: |  |  |  |  |
Asset retirement obligation capitalized | $ 9,505 | $ 3,195 |  |  |
Level 1 |  |  |  |  |
Assets: |  |  |  |  |
Derivative Instruments | 0 |  | $ 0 |  |
Liabilities: |  |  |  |  |
Derivative Instruments | 0 |  | 0 |  |
Level 2 |  |  |  |  |
Assets: |  |  |  |  |
Derivative Instruments | 66,177 |  | 9,184 |  |
Liabilities: |  |  |  |  |
Derivative Instruments | 36,304 |  | 145,978 |  |
Level 3 |  |  |  |  |
Assets: |  |  |  |  |
Derivative Instruments | 0 |  | 0 |  |
Liabilities: |  |  |  |  |
Derivative Instruments | 0 |  | $ 0 |  |
Asset retirement obligation capitalized | $ 9,500 |  |  |  |
Level 3 | Investment in Grizzly Oil Sands ULC |  |  |  |  |
Liabilities: |  |  |  |  |
Equity investment |  |  |  | $ 39,100 |
Level 3 | Investment in Strike Force Midstream LLC |  |  |  |  |
Liabilities: |  |  |  |  |
Equity investment |  |  |  |  | $ 22,500

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