Major classes of assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Current assets:
Receivables, net	$ 33,551	$ 94,132	$ 146,589
Inventories	6,748	11,401	12,849
Deferred income taxes	0	0	6,623
Commodity derivative instruments	2,537	18,335	129
Prepayments and other current assets	34,456	7,309	6,328
Total current assets held for sale	77,292	131,177	172,518
Noncurrent assets:
Investments	37	37	37
Net property, plant and equipment	1,097,576	1,618,099	1,753,509
Deferred income taxes	52,017	0	0
Other	161	2,334	4,091
Less allowance for impairment of assets held for sale	399,987	0	0
Total noncurrent assets held for sale	749,804	1,620,470	1,757,637
Total assets held for sale	827,096	1,751,647	1,930,155
Current liabilities:
Long-term debt due within one year	0	897	569
Accounts payable	49,400	103,556	165,189
Taxes payable	4,064	19,900	15,051
Deferred income taxes	1,401	8,206	0
Accrued compensation	4,460	5,373	5,721
Commodity derivative instruments	3,511	0	17,449
Other accrued liabilities	12,107	16,796	27,640
Total current liabilities held for sale	74,943	154,728	231,619
Noncurrent liabilities:
Long-term debt	0	0	608
Deferred income taxes	0	238,391	257,316
Other liabilities	35,790	57,050	60,761
Total noncurrent liabilities held for sale	35,790	295,441	318,685
Total liabilities held for sale	$ 110,733	$ 450,169	$ 550,304