Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of quarterly financial information
Revenues	$ 121,524	$ 119,799	$ 116,317	$ 112,657	$ 113,168	$ 113,462	$ 111,634	$ 112,124	$ 470,298	$ 450,389	$ 424,737
Net income attributable to common stockholders	$ 27,210	$ 2,601	$ 4,484	$ 4,891	$ 16,314	$ 6,548	$ 37,729	$ 9,180
Net income attributable to common stockholders per share
Basic earnings per common share (in USD per share)	$ 0.20	$ 0.02	$ 0.03	$ 0.04	$ 0.13	$ 0.05	$ 0.30	$ 0.07	$ 0.29	$ 0.55	$ 0.18
Diluted earnings per common share (in USD per share)	$ 0.20	$ 0.02	$ 0.03	$ 0.04	$ 0.13	$ 0.05	$ 0.30	$ 0.07	$ 0.29	$ 0.55	$ 0.18