Selected Quarterly Financial Data (Details) - USD ($)	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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 35,628,000		$ 35,912,000		$ 35,660,000		$ 36,883,000		$ 36,932,000		$ 36,170,000		$ 41,350,000		$ 37,568,000		$ 144,083,000	$ 152,020,000	$ 146,008,000
Net income (loss)	(9,914,000)		2,947,000		5,544,000		2,989,000		4,736,000		6,305,000		9,937,000		(16,620,000)		1,566,000	4,358,000	19,653,000
Net income (loss) attributable to common shareholders	$ (12,657,000)		$ 92,000		$ 2,695,000		$ 194,000		$ 1,932,000		$ 3,472,000		$ 7,089,000		$ (22,974,000)		$ (9,676,000)	$ (10,481,000)	$ (2,399,000)
Per common share (basic and diluted)	$ (0.15)	[1]	$ 0.00	[1]	$ 0.03	[1]	$ 0.00	[1]	$ 0.02	[1]	$ 0.04	[1]	$ 0.08	[1]	$ (0.26)	[1]	$ (0.12)	$ (0.13)	$ (0.04)

[1] Differences between the sum of the four quarterly per share amounts and the annual per share amounts, if any, are attributable to the effect of the weighted average outstanding share calculation for the respective periods.