Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 31,786	$ 31,080
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	24,098	24,154
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	4,873	5,015
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 2,815	$ 1,911