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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