Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 28,354	$ 26,321	$ 89,462	$ 68,322
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	20,631	17,029	65,336	45,081
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	4,752	6,450	14,235	17,462
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 2,971	$ 2,842	$ 9,891	$ 5,779