Equity Method Investments - Distributions received, by investment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital	$ 18,640	$ 23,257	$ 57,219	$ 70,902
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	18,640	19,847	57,219	60,552
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	$ 0	$ 3,410	$ 0	$ 10,350