Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly financial data (unaudited)
Revenue	$ 67,808	$ 65,253	$ 59,601	$ 58,159	$ 57,348	$ 53,655	$ 49,033	$ 44,333	$ 250,821	$ 204,369	$ 159,344
(Loss) income from operations	(3,157)	150	2,576	(2,566)	(3,464)	(2,989)	(7,670)	(6,578)	(2,997)	(20,701)	(26,097)
Net (loss) income attributable to common stockholders	$ 416	$ (522)	$ 2,115	$ (3,229)	$ (1,019)	$ (3,450)	$ (8,017)	$ (6,880)	$ (1,220)	$ (19,366)	$ (27,331)
Basic and diluted (loss) income per share (in dollars per share)	$ 0.01	$ (0.01)	$ 0.05	$ (0.08)	$ (0.02)	$ (0.09)	$ (0.20)	$ (0.18)	$ (0.03)	$ (0.49)	$ (0.72)