Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 24,782	$ 30,028	$ 56,568	$ 61,108
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	21,664	20,551	45,762	44,705
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	0	4,468	4,873	9,483
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Cash distributions received from equity method investments	$ 3,118	$ 5,009	$ 5,933	$ 6,920