Investments in Non-Consolidated Subsidiaries (Income Statement Disclosure) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2010 White Cliffs Pipeline, LLC [Member]	Dec. 31, 2012 White Cliffs Pipeline, LLC [Member]	Dec. 31, 2011 White Cliffs Pipeline, LLC [Member]	Sep. 30, 2012 Ngl Energy Partners Lp [Member]
Summarized income statement information
Revenue				$ 13,619	$ 108,125	$ 66,097	$ 2,371,524
Costs of products sold	874,885	1,144,439	1,258,695				2,182,263
Operating, general and administrative expenses				3,294	14,821	12,746	125,889
Depreciation and amortization expense				5,680	19,963	20,842	34,621
Net income				4,645	73,341	32,509	5,405
Distributions paid to SemGroup				$ 5,768	$ 44,514	$ 27,459