Reconciliation of income and expenses (Details 5) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations	$ (118)	$ (2,198)		$ 85	$ (3,702)
Discontinued operations, held-for-sale or disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	61	121		201	356
Operating expenses	216	384		825	(4,988)
Operating income (loss)	(155)	(263)		(624)	5,344
Other income	0	0		12
Other expense					(13)
Interest expense	0	0		575	239
Income (loss) from discontinued operations before income taxes	(155)	(263)		(1,187)	5,092
Income taxes	(37)	1,935		(1,272)	8,794
Income (loss) from discontinued operations	$ (118)	$ (2,198)		$ 85	$ (3,702)
Reversal of accrual | Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating expenses			$ 7,000
Reversal of accrual - after tax | Refining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating expenses			$ 4,300