Major classes of assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Current assets:
Receivables, net	$ 24,703	$ 94,132	$ 117,098
Inventories	7,034	11,401	13,504
Commodity derivative instruments	8,633	18,335	11,322
Prepayments and other current assets	42,762	7,309	2,114
Total current assets held for sale	83,132	131,177	144,038
Noncurrent assets:
Investments	37	37	37
Net property, plant and equipment	1,114,285	1,618,099	1,609,385
Deferred income taxes	141,556	0	0
Other	162	2,334	2,635
Less allowance for impairment of assets held for sale	756,127	0	0
Total noncurrent assets held for sale	499,913	1,620,470	1,612,057
Total assets held for sale	583,045	1,751,647	1,756,095
Current liabilities:
Long-term debt due within one year	0	897	528
Accounts payable	32,375	103,556	141,877
Taxes payable	3,769	19,900	39,693
Deferred income taxes	4,955	8,206	4,005
Accrued compensation	5,982	5,373	6,622
Commodity derivative instruments	0	0	44
Other accrued liabilities	11,820	16,796	27,342
Total current liabilities held for sale	58,901	154,728	220,111
Noncurrent liabilities:
Long-term debt	0	0	510
Deferred income taxes	0	238,391	212,640
Other liabilities	31,242	57,050	58,162
Total noncurrent liabilities held for sale	31,242	295,441	271,312
Total liabilities held for sale	$ 90,143	$ 450,169	$ 491,423