Disposal of long-lived assets (Details) - USD ($) $ in Thousands			3 Months Ended
	Apr. 12, 2018	Mar. 15, 2018	Mar. 31, 2018	Dec. 31, 2017
Disposal of long-lived assets [Line Items]
Current assets held for sale			$ 2,501	$ 38,063
Current liabilities held for sale			2,434	$ 23,847
SemMexico [Member]
Disposal of long-lived assets [Line Items]
Proceeds from Divestiture of Businesses		$ 73,500
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			4,400
Income (loss) from disposal, excluding discontinued operations, before tax, excluding gain/loss on disposal			2,300
SemLogistics [Member]
Disposal of long-lived assets [Line Items]
Income (loss) from disposal, excluding discontinued operations, before tax, excluding gain/loss on disposal			5,800
Subsequent Event [Member] | SemLogistics [Member]
Disposal of long-lived assets [Line Items]
Proceeds from Divestiture of Businesses	$ 76,800
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | SemLogistics [Member]
Disposal of long-lived assets [Line Items]
Disposal Group, Including Discontinued Operations, Net Realizable Value			75,100
Impairment of Long-Lived Assets to be Disposed of			200
Disposal Group, Including Discontinued Operation, Property, Plant and Equipment			142,600
Current assets held for sale			2,500
Current liabilities held for sale			$ 2,400