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)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Net proceeds from follow-on from public offering $ 67,840
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 | shares 4,600
Public offerings of common stock, Price Per Share | $ / shares $ 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 | shares 65
Public offerings of common stock, Price Per Share | $ / shares $ 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 | shares 4,025
Public offerings of common stock, Price Per Share | $ / shares $ 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 | shares 407
Public offerings of common stock, Price Per Share | $ / shares $ 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 | shares 197
Public offerings of common stock, Price Per Share | $ / shares $ 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 | shares 3,450
Public offerings of common stock, Price Per Share | $ / shares $ 18.73
Net proceeds from follow-on from public offering $ 64,000