Reporting Segments - Reconciliation of Segment Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Reconciliation of segment operating income to net income (loss):
Segment operating income	$ 10,914	$ 373
General and administrative	(5,165)	(5,609)
Unit-based compensation expense	(1,438)	(540)
Interest expense, net	(2,599)	(1,883)
Other income (expense)	(270)
Net income (loss)	$ 1,442	$ (7,659)	$ (3,040)