Quarterly Financial Data Quarterly Financial Data (Details Textual) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SemGas [Member]
Goodwill, Impairment Loss					$ 13,052
Crude Transportation [Member]
Goodwill, Impairment Loss		$ 26,600		$ 26,628		$ 9,488
Impairment of Intangible Assets, Finite-lived	$ 12,100			12,087
Kansas gas gathering and compression assets [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Gain (loss) on disposal			$ 4,500
SemMexico [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Impairment of Long-Lived Assets to be Disposed of	13,500			13,511
Sem Logistics [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Impairment of Long-Lived Assets to be Disposed of	76,700			76,661
Glass Mountain Pipeline Llc [Member]
Equity Method Investment, Realized Gain (Loss) on Disposal	$ 150,300			$ 150,266