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Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Current assets:
Total current assets held for sale	$ 304	$ 14,391	$ 45,867
Noncurrent assets:
Total noncurrent assets held for sale	$ 64,333	196,664	$ 112,440
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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