Equity - Class of Stock Disclosures - Common Stock and Shelf Registrations (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity
Proceeds from common stock issued, net of issuance costs		$ 1,005,000	$ 14,578,000	$ 78,592,000
Common Stock
Equity
Number of shares repurchased		31,326	42,089	49,405
Common Stock | Subsequent Event
Equity
Number of shares repurchased	20,345
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	$ 14,600,000
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