Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 30,028	$ 23,075	$ 61,108	$ 42,001
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	20,551	14,467	44,705	28,052
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	4,468	5,671	9,483	11,012
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 5,009	$ 2,937	$ 6,920	$ 2,937