Equity Method Investments - Investment balances (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 430,514	$ 434,289
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	273,868	281,734
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	18,942	18,933
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 137,704	$ 133,622