Equity - Class of Stock Disclosures - Common Stock and Shelf Registrations (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Equity
Proceeds from common stock issued, net of issuance costs	$ 1,005,000	$ 14,578,000
Common Stock
Equity
Number of shares repurchased	31,326	41,592
Common Stock | Equity Distribution Agreements
Equity
Maximum offering capacity under shelf registration statement	$ 200,000,000
Shares common stock sold	22,244	312,881
Proceeds from common stock issued, net of issuance costs	$ 1,005,000	$ 14,578,000
Compensation paid to sales agents	18,000	260,000
Reclassification of accumulated costs to additional paid in capital	76,000	49,000
Amount available under effective shelf registration statement	$ 184,139,000	$ 185,162,000