Major classes of assets and liabilities held for sale (Details 4) - USD ($) $ in Thousands		Sep. 30, 2017		Dec. 31, 2016		Sep. 30, 2016
Current assets:
Total current assets held for sale		$ 304		$ 14,391		$ 45,867
Noncurrent assets:
Total noncurrent assets held for sale		64,333		196,664		112,440
Current liabilities:
Total current liabilities held for sale		5,749		9,924		18,065
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net		304		355		7,930
Total current assets held for sale		304		355		7,930
Noncurrent assets:
Net property, plant and equipment		2,064		5,507		5,507
Deferred income taxes		62,163		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		64,388		95,828		109,456
Total assets held for sale		64,692		96,183		117,386
Current liabilities:
Accounts payable		68		141		175
Taxes payable		11,745		19	[1]	2,205	[1]
Other accrued liabilities		2,380		2,358		3,084
Total current liabilities held for sale		14,193		2,518		5,464
Deferred credits and other liabilities:
Total liabilities held for sale		14,193		2,518		5,464
Refining [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net		0		0		13
Income taxes receivable		8,444	[2]	13,987		32,388
Prepayments and other current assets		0		0		7,741
Total current assets held for sale		8,444		13,987		40,142
Noncurrent assets:
Deferred income taxes		0		0		2,984
Total noncurrent assets held for sale		0		0		2,984
Total assets held for sale		8,444		13,987		43,126
Current liabilities:
Accounts payable		0		7,425		7,063
Other accrued liabilities		0		0		7,743
Total current liabilities held for sale		0		7,425		14,806
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		$ 14,806

[1] On the Company's Consolidated Balance Sheets, these amounts were reclassified to prepayments and other current assets and are reflected in 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.