Investments in Non-Consolidated Subsidiaries (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 White Cliffs Pipeline, L.L.C. [Member]	Sep. 30, 2011 White Cliffs Pipeline, L.L.C. [Member]	Sep. 30, 2012 White Cliffs Pipeline, L.L.C. [Member]	Sep. 30, 2011 White Cliffs Pipeline, L.L.C. [Member]	Jun. 30, 2012 NGL Energy Partners LP [Member]	Jun. 30, 2012 NGL Energy Partners LP [Member]
Summarized income statement information
Revenue					$ 28,522	$ 17,515	$ 76,910	$ 47,878	$ 326,436	$ 1,236,023
Cost of products sold	189,830	313,490	651,283	896,871					298,985	1,128,581
Operating, general and administrative expenses					3,857	3,824	11,382	10,029	33,298	76,015
Depreciation and amortization expense					4,995	5,214	14,964	15,622	9,227	21,260
Net income					19,670	8,477	50,564	22,227	(24,710)	(4,678)
Distributions paid to SemGroup					$ (10,794)	$ (7,239)	$ (30,561)	$ (19,912)