Equity Method Investments - Investment balances (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 432,389	$ 434,289
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	279,333	281,734
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	18,936	18,933
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 134,120	$ 133,622