Quarterly report pursuant to Section 13 or 15(d)

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

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Equity - Schedule of Common Stock Public Offerings and ATM (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Net proceeds from follow-on from public offering $ 98,316 $ 91,167
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  
11/09/2016 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 Nov. 09, 2016  
Public offerings of common stock, Shares Issued 4,025  
Public offerings of common stock, Price Per Share $ 19.28  
Net proceeds from follow-on from public offering $ 77,000  
12/13/16 to 12/29/16 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 407  
Public offerings of common stock, Price Per Share $ 19.47  
Net proceeds from follow-on from public offering $ 8,000  
12/13/16 to 12/29/16 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 Dec. 13, 2016  
12/13/16 to 12/29/16 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 Dec. 29, 2016  
1/20/17 to 2/2/17 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 197  
Public offerings of common stock, Price Per Share $ 19.18  
Net proceeds from follow-on from public offering $ 4,000  
1/20/17 to 2/2/17 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 Jan. 20, 2017  
1/20/17 to 2/2/17 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 Feb. 02, 2017  
3/10/2017 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 Mar. 10, 2017  
Public offerings of common stock, Shares Issued 3,450  
Public offerings of common stock, Price Per Share $ 18.73  
Net proceeds from follow-on from public offering $ 64,000  
5/17/17 to 6/22/17 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 1,376  
Public offerings of common stock, Price Per Share $ 22.71  
Net proceeds from follow-on from public offering $ 31,000  
5/17/17 to 6/22/17 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 17, 2017  
5/17/17 to 6/22/17 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. 22, 2017