Quarterly report pursuant to Section 13 or 15(d)

Equity - Schedule of Common Stock Public Offerings and ATM Offerings (Detail)

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Equity - Schedule of Common Stock Public Offerings and ATM Offerings (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Net proceeds from follow-on from public offering $ 91,516 $ 81,540
5/4/2015 Closing Date [Member] | Public Offering [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Public offerings and ATM offerings of common stock, Closing Date May 04, 2015  
Public offerings of common stock, Shares Issued 4,600  
Public offerings of common stock, Price Per Share $ 18.50  
Net proceeds from follow-on from public offering $ 82,000  
10/19/2015 Closing Date [Member] | Public Offering [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Public offerings and ATM offerings of common stock, Closing Date Oct. 19, 2015  
Public offerings of common stock, Shares Issued 5,750  
Public offerings of common stock, Price Per Share $ 18.00  
Net proceeds from follow-on from public offering $ 99,000  
6/21/2016 Closing Date [Member] | Public Offering [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Public offerings and ATM offerings of common stock, Closing Date Jun. 21, 2016  
Public offerings of common stock, Shares Issued 4,600  
Public offerings of common stock, Price Per Share $ 19.78  
Net proceeds from follow-on from public offering $ 91,000  
5/09/2016 to 6/30/2016 Closing Date [Member] | At the Market Offering [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Public offerings of common stock, Shares Issued 65  
Public offerings of common stock, Price Per Share $ 20.31  
Net proceeds from follow-on from public offering $ 1,000  
5/09/2016 to 6/30/2016 Closing Date [Member] | Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Public offerings and ATM offerings of common stock, Closing Date May 09, 2016  
5/09/2016 to 6/30/2016 Closing Date [Member] | Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Public offerings and ATM offerings of common stock, Closing Date Jun. 30, 2016