Equity Method Investments - Equity earnings, 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]
Earnings from equity method investments	$ 17,753	$ 17,078	$ 34,844	$ 40,149
White Cliffs Pipeline, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	15,976	16,428	31,169	36,208
NGL Energy Partners LP [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	6	0	9	2,232
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 1,771	$ 650	$ 3,666	$ 1,709