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Stockholders' Equity - Equity Offerings (Detail) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 3 Months Ended 5 Months Ended 10 Months Ended 12 Months Ended
Mar. 31, 2018
May 31, 2017
Jun. 30, 2016
Dec. 31, 2017
May 23, 2017
Dec. 31, 2016
Dec. 31, 2018
September 2017 At-The-Market              
Equity offerings              
Issuance of common stock, net of issuance costs, shares       1,600,000      
Weighted average price per share issued       $ 12.50      
Aggregate net proceeds from stock sale       $ 19.3      
Common stock available for sale under agreement             $ 80.0
Securities sold during period             0
March 2016 At-The-Market              
Equity offerings              
Issuance of common stock, net of issuance costs, shares         1,388,647 668,791  
Weighted average price per share issued         $ 5.55 $ 6.336  
Aggregate net proceeds from stock sale         $ 7.4 $ 3.9  
Public Equity Offering              
Equity offerings              
Issuance of common stock, net of issuance costs, shares 8,050,000 8,050,000 3,476,793        
Weighted average price per share issued $ 17.00 $ 11.50 $ 6.10        
Aggregate net proceeds from stock sale $ 128.4 $ 86.8 $ 19.7