Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015		Jun. 30, 2015
Current assets:
Total current assets held for sale	$ 85,124	$ 54,847		$ 147,162
Noncurrent assets:
Total noncurrent assets held for sale	123,721	565,509		1,160,657
Current liabilities:
Total current liabilities held for sale	32,357	130,375		145,211
Noncurrent liabilities:
Total noncurrent liabilities held for sale	0	63,750		101,790
Refining [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Cash and cash equivalents	0	688		845
Receivables, net	433	7,693		29,639
Inventories	0	13,176		24,166
Deferred income taxes	0	0		84	[1]
Income taxes receivable	12,550	2,495		7,332
Prepayments and other current assets	11,083	6,214		7,888
Total current assets held for sale	24,066	30,266		69,954
Noncurrent assets:
Net property, plant and equipment	0	412,717		418,885
Deferred income taxes	57,644	5,745		5,839
Other	0	9,627		5,729
Total noncurrent assets held for sale	57,644	428,089		430,453
Total assets held for sale	81,710	458,355		500,407
Current liabilities:
Short-term borrowings	0	45,500		26,000
Long-term debt due within one year	0	5,250		3,000
Accounts payable	7,170	24,468		38,170
Taxes payable	0	1,391		1,601
Deferred income taxes	0	272		0
Accrued compensation	0	938		649
Other accrued liabilities	8,303	4,953		932
Total current liabilities held for sale	15,473	82,772		70,352
Noncurrent liabilities:
Long-term debt	0	63,750		66,000
Deferred income taxes	0	29,314	[2]	19,600	[2]
Total noncurrent liabilities held for sale	0	93,064		85,600
Total liabilities held for sale	15,473	175,836		155,952
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net	8,207	13,387		33,551
Inventories	0	1,308		6,748
Commodity derivative instruments	0	0		2,537
Income taxes receivable	52,847	9,665		31,033
Prepayments and other current assets	4	221		3,423
Total current assets held for sale	61,058	24,581		77,292
Noncurrent assets:
Investments	0	37		37
Net property, plant and equipment	5,507	793,422		1,097,576
Deferred income taxes	61,347	127,655		52,017
Other	161	161		161
Less allowance for impairment of assets held for sale	938	754,541		399,987
Total noncurrent assets held for sale	66,077	166,734		749,804
Total assets held for sale	127,135	191,315		827,096
Current liabilities:
Accounts payable	456	25,013		49,400
Taxes payable	0	1,052		4,064
Deferred income taxes	4,120	3,620		1,401
Accrued compensation	1,459	13,080		4,460
Commodity derivative instruments	0	0		3,511
Other accrued liabilities	10,849	4,838		12,107
Total current liabilities held for sale	16,884	47,603		74,943
Noncurrent liabilities:
Other liabilities	0	0		35,790
Total noncurrent liabilities held for sale	0	0		35,790
Total liabilities held for sale	$ 16,884	$ 47,603		$ 110,733

[1] On the Company's Consolidated Balance Sheet, this amount was reclassified to a current deferred income tax liability and is reflected in current liabilities held for sale.

[2] 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.