Equity Method Investments - Distributions received, by investment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 31,080	$ 18,926
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	24,154	13,585
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	5,015	5,341
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 1,911	$ 0