Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital	$ 25,951	$ 24,782	$ 47,644	$ 56,568
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital	22,514	21,664	40,704	45,762
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital	0	0	0	4,873
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital	$ 3,437	$ 3,118	$ 6,940	$ 5,933