Equity Method Investments - Equity earnings, by investment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016		Mar. 31, 2015
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 23,071		$ 20,559
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	19,780		19,090
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	2,232	[1]	(305)
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 1,059		$ 1,774

[1] * Excluding loss on issuance of common units of $41.0 thousand for the three months ended March 31, 2016