Major classes of assets and liabilities held for sale (Details 4) - USD ($) $ in Thousands		Jun. 30, 2017		Dec. 31, 2016		Jun. 30, 2016
Current assets:
Total current assets held for sale		$ 328		$ 14,391		$ 37,625
Noncurrent assets:
Total noncurrent assets held for sale		76,183		196,664		167,100
Current liabilities:
Total current liabilities held for sale		4,481		9,924		28,237
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net		328		355		8,207
Income taxes receivable		0		0		5,348
Prepayments and other current assets		0		0		4
Total current assets held for sale		328		355		13,559
Noncurrent assets:
Net property, plant and equipment		2,064		5,507		5,507
Deferred income taxes		74,013		91,098		104,726
Other		161		161		161
Less allowance for impairment of assets held for sale		0		938		938
Total noncurrent assets held for sale		76,238		95,828		109,456
Total assets held for sale		76,566		96,183		123,015
Current liabilities:
Accounts payable		138		141		456
Taxes payable		7,171		19	[1]	0
Accrued compensation		0		0		1,459
Other accrued liabilities		2,724		2,358		10,849
Total current liabilities held for sale		10,033		2,518		12,764
Deferred credits and other liabilities:
Total liabilities held for sale		10,033		2,518		12,764
Refining [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net		0		0		433
Income taxes receivable		5,552	[2]	13,987		12,550
Prepayments and other current assets		0		0		11,083
Total current assets held for sale		5,552		13,987		24,066
Noncurrent assets:
Deferred income taxes		0		0		57,644
Total noncurrent assets held for sale		0		0		57,644
Total assets held for sale		5,552		13,987		81,710
Current liabilities:
Accounts payable		0		7,425		7,170
Other accrued liabilities		0		0		8,303
Total current liabilities held for sale		0		7,425		15,473
Deferred credits and other liabilities:
Deferred income taxes (b)	[3]	55		14		0
Total noncurrent liabilities held for sale		55		14		0
Total liabilities held for sale		$ 55		$ 7,439		$ 15,473

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

[2] On the Company's Consolidated Balance Sheets, this amount was reclassified to income taxes payable and is reflected in current liabilities held for sale.

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