Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Current assets:
Total current assets held for sale	$ 328	$ 14,391	$ 37,625
Noncurrent assets:
Total noncurrent assets held for sale	$ 76,183	196,664	$ 167,100
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