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Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2016
--- | --- | --- | ---
Current assets: |  |  |
Total current assets held for sale | $ 7,290 | $ 14,391 | $ 99,544
Noncurrent assets: |  |  |
Total noncurrent assets held for sale | $ 95,719 | 196,664 | $ 485,885
Pronghorn [Member] | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member] |  |  |
Current assets: |  |  |
Prepayments and other current assets |  | 68 |
Total current assets held for sale |  | 68 |
Noncurrent assets: |  |  |
Net property, plant and equipment |  | 93,424 |
Goodwill |  | 9,737 |
Less allowance for impairment of assets held for sale |  | 2,311 |
Total noncurrent assets held for sale |  | 100,850 |
Total assets held for sale |  | $ 100,918 |

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