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Equity - Class of Stock Disclosures - Common Stock and Shelf Registrations (Details) - USD ($)
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Equity    
Net proceeds   $ 14,578,000
Common Stock    
Equity    
Number of shares repurchased 28,256 35,563
Common Stock | Equity Distribution Agreements    
Equity    
Maximum offering capacity under shelf registration statement $ 200,000,000  
Shares common stock sold   312,881
Net proceeds   $ 14,578,000
Amount available under effective shelf registration statement $ 185,162,000 $ 185,162,000