Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Equity - Schedule of Common Stock Public Offerings and ATM (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Net proceeds from follow-on from public offering $ 96,899 $ 177,294 $ 180,486
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