Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital	$ 21,693	$ 31,786
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	18,190	24,098
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	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	$ 3,503	$ 2,815