Equity - Equity rollforward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
SemGroup owners’ equity:
Balance at December 31, 2017	$ 1,658,365
Adoption of ASC 606	11,513
Net loss	(33,035)	$ (10,277)
Other comprehensive income, net of income tax	18,171	$ 6,033
Dividends paid	(37,230)
Unvested dividend equivalent rights	53
Non-cash equity compensation	2,149
Issuance of common stock under compensation plans	558
Retirement of treasury stock	0
Repurchase of common stock	(381)
Balance at March 31, 2018	1,620,163
Common Stock [Member]
SemGroup owners’ equity:
Balance at December 31, 2017	786
Adoption of ASC 606	0
Issuance of common stock under compensation plans	1
Retirement of treasury stock	(2)
Balance at March 31, 2018	785
Additional Paid-In Capital [Member]
SemGroup owners’ equity:
Balance at December 31, 2017	1,770,117
Adoption of ASC 606	0
Dividends paid	(37,230)
Unvested dividend equivalent rights	53
Non-cash equity compensation	2,149
Issuance of common stock under compensation plans	557
Retirement of treasury stock	0
Balance at March 31, 2018	1,735,646
Treasury Stock [Member]
SemGroup owners’ equity:
Balance at December 31, 2017	(8,031)
Adoption of ASC 606	0
Retirement of treasury stock	8,031
Repurchase of common stock	(381)
Balance at March 31, 2018	(381)
Accumulated Deficit [Member]
SemGroup owners’ equity:
Balance at December 31, 2017	(50,706)
Adoption of ASC 606	11,513
Net loss	(33,035)
Retirement of treasury stock	(8,029)
Balance at March 31, 2018	(80,257)
Accumulated Other Comprehensive Income (Loss) [Member]
SemGroup owners’ equity:
Balance at December 31, 2017	(53,801)
Adoption of ASC 606	0
Retirement of treasury stock	0
Balance at March 31, 2018	$ (35,630)