Selected Quarterly Financial Data (Details) - USD ($)	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 37,170,000		$ 37,793,000		$ 37,872,000		$ 38,251,000		$ 37,730,000		$ 36,100,000		$ 36,742,000		$ 38,635,000		$ 151,086,000	$ 149,207,000	$ 148,184,000
Net income (loss)	3,154,000		(761,000)		4,795,000		1,576,000		6,840,000		6,126,000		5,416,000		3,399,000		8,764,000	21,781,000	28,691,000
Net (loss) income attributable to common shareholders	$ (523,000)		$ (4,289,000)		$ 1,286,000		$ (1,939,000)		$ 3,356,000		$ 2,590,000		$ 1,915,000		$ (123,000)		$ (5,465,000)	$ 7,738,000	$ 14,573,000
Per common share (basic and diluted)	$ (0.01)	[1]	$ (0.05)	[1]	$ 0.01	[1]	$ (0.03)	[1]	$ 0.04	[1]	$ 0.03	[1]	$ 0.02	[1]	$ 0.00	[1]	$ (0.08)	$ 0.09	$ 0.18

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