Equity - Class of Stock Disclosures - Common Stock and Shelf Registrations (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity
Net proceeds		$ 14,578,000	$ 78,592,000
Common Stock
Equity
Number of shares repurchased		42,089	49,405	26,993
Common Stock | Subsequent Event
Equity
Number of shares repurchased	28,256
Common Stock | Original Agreement
Equity
Maximum offering capacity under shelf registration statement		$ 200,000,000
Shares common stock sold		312,881	1,643,017	0
Net proceeds		$ 14,600,000	$ 78,600,000
Compensation paid to sales agents		260,000	1,400,000
Reclassification of accumulated costs to additional paid in capital		49,000	$ 463,000
Amount available under effective shelf registration statement		$ 185,162,000