Equity Method Investments - Investment balances (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 503,914	$ 551,078
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	293,811	297,109
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	70,342	112,787
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 139,761	$ 141,182