Major classes of assets and liabilities (Details 4) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015
Current assets:
Total current assets held for sale		$ 14,391		$ 54,847
Noncurrent assets:
Total noncurrent assets held for sale		196,664		561,617
Current liabilities:
Total current liabilities held for sale		9,924		126,483
Noncurrent liabilities:
Total noncurrent liabilities held for sale		0		63,750
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Current assets:
Receivables, net		355		13,387
Inventories		0		1,308
Income taxes receivable		0		9,665
Prepayments and other current assets		0		221
Total current assets held for sale		355		24,581
Noncurrent assets:
Investments		0		37
Net property, plant and equipment		5,507		793,422
Deferred income taxes		91,098		124,035
Other		161		161
Less allowance for impairment of assets held for sale		938		754,541
Total noncurrent assets held for sale		95,828		163,114
Total assets held for sale		96,183		187,695
Current liabilities:
Accounts payable		141		25,013
Taxes payable		19	[1]	1,052
Accrued compensation		0		13,080
Other accrued liabilities		2,358		4,838
Total current liabilities held for sale		2,518		43,983
Noncurrent liabilities:
Total liabilities held for sale		2,518		43,983
Refining [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Current assets:
Cash and cash equivalents		0		688
Receivables, net		0		7,693
Inventories		0		13,176
Income taxes receivable		13,987		2,495
Prepayments and other current assets		0		6,214
Total current assets held for sale		13,987		30,266
Noncurrent assets:
Net property, plant and equipment		0		412,717
Other		0		9,627
Total noncurrent assets held for sale		0		422,344
Total assets held for sale		13,987		452,610
Current liabilities:
Short-term borrowings		0		45,500
Long-term debt due within one year		0		5,250
Accounts payable		7,425		24,468
Taxes payable		0		1,391
Accrued compensation		0		938
Other accrued liabilities		0		4,953
Total current liabilities held for sale		7,425		82,500
Noncurrent liabilities:
Long-term debt		0		63,750
Deferred income taxes	[2]	14		23,841
Total noncurrent liabilities held for sale		14		87,591
Total liabilities held for sale		$ 7,439		$ 170,091

[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.

[2] On the Company's Consolidated Balance Sheets, these amounts were reclassified to noncurrent deferred income tax assetsand are reflected in noncurrent assets held for sale.