Equity Method Investments - Equity earnings, 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]
Earnings from equity method investments	$ 14,528	$ 17,367	$ 41,493	$ 52,211
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	14,546	15,636	41,489	46,805
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	0	1,736	0	5,402
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ (18)	$ (5)	$ 4	$ 4