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Shareholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended 15 Months Ended
Oct. 10, 2016
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2017
Cowen and Company, LLC        
Shareholders' Equity        
ATM Agreement, proceeds from issuance of common stock     $ 8.0  
Commission and issuance cost     $ 0.3  
Cowen and Company, LLC | Common Stock        
Shareholders' Equity        
ATM Agreement, shares issued (in shares)       2,340,879
Cowen and Company, LLC | Maximum        
Shareholders' Equity        
Percentage of gross proceeds payable as commission (up to) 3.00%      
Public Stock Offering        
Shareholders' Equity        
Commission and issuance cost   $ 4.7    
Sale of Stock, Consideration Received on Transaction   $ 70.1    
Public Stock Offering | Common Stock        
Shareholders' Equity        
Sale of Stock, Number of Shares Issued in Transaction   5,750,000    
Sale of Stock, Price Per Share   $ 13.00