Selected Unaudited Quarterly Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected Unaudited Quarterly Information
Revenue	$ 59,810	$ 62,247	$ 60,808	$ 62,983	$ 68,997	$ 68,539	$ 66,813	$ 64,716
Net income	$ 37,440	$ (1,679)	$ (2,075)	$ (1,071)	$ 3,648	$ 2,399	$ 1,633	$ (1,205)	$ 32,615	$ 6,475	$ 13,069
Basic and diluted net income (loss) per share	$ 0.35	$ (0.02)	$ (0.02)	$ (0.01)	$ 0.03	$ 0.02	$ 0.02	$ (0.01)	$ 0.30	$ 0.06	$ 0.12
Weighted average number of shares outstanding	107,328,000	107,328,000	107,287,000	107,269,000	107,240,000	107,231,000	107,231,000	107,231,000	107,303,000	107,233,000	107,231,000