Equity Net income applicable to partners (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Net income attributable to partners	$ 90,182	$ 69,409	$ 62,804
General partner's IDRs	(24,224)	(17,389)	(11,160)
Net income available to partners	$ 65,958	$ 52,020	$ 51,644
General partner's ownership interest	2.00%	2.00%	2.00%
General partner's allocated interest in net income	$ 1,319	$ 1,040	$ 1,033
Total general partner's interest in net income	$ 25,543	$ 18,429	$ 12,193