Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Total current assets held for sale	$ 24,581	$ 131,177
Noncurrent assets:
Total noncurrent assets held for sale	166,734	1,620,470
Current liabilities:
Total current liabilities held for sale	47,603	154,728
Noncurrent liabilities:
Total noncurrent liabilities held for sale	0	295,441
Discontinued operations, held-for-sale or disposed by sale [Member]
Current assets:
Receivables, net	13,387	94,132
Inventories	1,308	11,401
Commodity derivative instruments	0	18,335
Prepayments and other current assets	9,886	7,309
Total current assets held for sale	24,581	131,177
Noncurrent assets:
Investments	37	37
Net property, plant and equipment	793,422	1,618,099
Deferred income taxes	127,655	0
Other	161	2,334
Less allowance for impairment of assets held for sale	754,541	0
Total noncurrent assets held for sale	166,734	1,620,470
Total assets held for sale	191,315	1,751,647
Current liabilities:
Long-term debt due within one year	0	897
Accounts payable	25,013	103,556
Taxes payable	1,052	19,900
Deferred income taxes	3,620	8,206
Accrued compensation	13,080	5,373
Other accrued liabilities	4,838	16,796
Total current liabilities held for sale	47,603	154,728
Noncurrent liabilities:
Deferred income taxes	0	238,391
Other liabilities	0	57,050
Total noncurrent liabilities held for sale	0	295,441
Total liabilities held for sale	$ 47,603	$ 450,169