Major classes of assets and liabilities held for sale (Details 5) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Total current assets held for sale	$ 479	$ 14,391
Deferred charges and other assets:
Total noncurrent assets held for sale	4,392	196,664
Liabilities
Total current liabilities held for sale	11,993	9,924
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Current assets:
Receivables, net	479	355
Total current assets held for sale	479	355
Deferred charges and other assets:
Net property, plant and equipment	1,631	5,507
Deferred income taxes	2,637	91,098
Other	161	161
Less allowance for impairment of assets held for sale	0	938
Total noncurrent assets held for sale	4,429	95,828
Total assets held for sale	4,908	96,183
Liabilities
Accounts payable	30	141
Taxes payable	10,857	19	[1]
Other accrued liabilities	2,884	2,358
Total current liabilities held for sale	13,771	2,518
Total liabilities held for sale	$ 13,771	$ 2,518

[1] On the Company's Consolidated Balance Sheets, this amount was reclassified to prepayments and other current assets and is reflected in current assets held for sale.