Major classes of assets and liabilities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015		Sep. 30, 2015
Current assets:
Total current assets held for sale	$ 93,366	$ 54,847		$ 117,823
Noncurrent assets:
Total noncurrent assets held for sale	69,061	565,509		909,150
Current liabilities:
Total current liabilities held for sale	22,185	130,375		123,628
Noncurrent liabilities:
Total noncurrent liabilities held for sale	0	63,750		96,117
Refining [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Cash and cash equivalents	0	688		564
Receivables, net	13	7,693		14,648
Inventories	0	13,176		12,354
Deferred income taxes	0	0		116	[1]
Income taxes receivable	32,388	2,495		0
Prepayments and other current assets	7,741	6,214		7,125
Total current assets held for sale	40,142	30,266		34,807
Noncurrent assets:
Net property, plant and equipment	0	412,717		415,817
Deferred income taxes	2,984	0		0
Other	0	9,627		5,052
Total noncurrent assets held for sale	2,984	422,344		420,869
Total assets held for sale	43,126	452,610		455,676
Current liabilities:
Short-term borrowings	0	45,500		29,500
Long-term debt due within one year	0	5,250		4,125
Accounts payable	7,063	24,468		21,472
Taxes payable	0	1,391		7,470
Deferred income taxes	0	272		0
Accrued compensation	0	938		1,059
Other accrued liabilities	7,743	4,953		1,217
Total current liabilities held for sale	14,806	82,772		64,843
Noncurrent liabilities:
Long-term debt	0	63,750		64,875
Deferred income taxes	0	23,569	[2]	11,632	[2]
Total noncurrent liabilities held for sale	0	87,319		76,507
Total liabilities held for sale	14,806	170,091		141,350
Exploration and production [Member] | Discontinued operations, held-for-sale or disposed of by sale [Member]
Current assets:
Receivables, net	7,930	13,387		24,703
Inventories	0	1,308		7,034
Commodity derivative instruments	0	0		8,633
Income taxes receivable	45,294	9,665		0
Prepayments and other current assets	0	221		42,762
Total current assets held for sale	53,224	24,581		83,132
Noncurrent assets:
Investments	0	37		37
Net property, plant and equipment	5,507	793,422		1,114,285
Deferred income taxes	61,347	127,655		141,556
Other	161	161		162
Less allowance for impairment of assets held for sale	938	754,541		756,127
Total noncurrent assets held for sale	66,077	166,734		499,913
Total assets held for sale	119,301	191,315		583,045
Current liabilities:
Accounts payable	175	25,013		32,375
Taxes payable	0	1,052		3,769
Deferred income taxes	4,120	3,620		4,955
Accrued compensation	0	13,080		5,982
Other accrued liabilities	3,084	4,838		11,820
Total current liabilities held for sale	7,379	47,603		58,901
Noncurrent liabilities:
Other	0	0		31,242
Total noncurrent liabilities held for sale	0	0		31,242
Total liabilities held for sale	$ 7,379	$ 47,603		$ 90,143

[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.