Revenue recognition - Comparison to Amounts if ASC 605 Had Been in Effect (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	$ 34,791	$ 35,180	$ 63,826	$ 68,339
Operating expenses	26,468	24,684	50,706	50,786
Operating income	8,323	$ 10,496	13,120	$ 17,553
ASU 2014-09 | ASC 605 Presentation
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	33,906		62,135
Operating expenses	25,583		49,015
Operating income	8,323		13,120
ASU 2014-09 | Reclassification
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	(885)		(1,691)
Operating expenses	$ (885)		$ (1,691)