Quarterly report pursuant to Section 13 or 15(d)

Equity - Schedule of Common Stock Public Offerings (Detail)

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Equity - Schedule of Common Stock Public Offerings (Detail)
$ / shares in Units, shares in Thousands, $ in Millions
 9 Months Ended
Sep. 30, 2015
USD ($)
$ / shares
shares
4/23/2013 Closing Date [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Public offerings of common stock, Closing Date Apr. 23, 2013
Public offerings of common stock, Shares Issued | shares 14,150
Public offerings of common stock, Price Per Share | $ / shares $ 12.50
Net proceeds from offering $ 160
4/29/2014 Closing Date [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Public offerings of common stock, Closing Date Apr. 29, 2014
Public offerings of common stock, Shares Issued | shares 5,750
Public offerings of common stock, Price Per Share | $ / shares $ 13.00
Net proceeds from follow-on from public offering $ 70
10/31/2014 Closing Date [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Public offerings of common stock, Closing Date Oct. 31, 2014
Public offerings of common stock, Shares Issued | shares 4,600
Public offerings of common stock, Price Per Share | $ / shares $ 13.60
Net proceeds from follow-on from public offering $ 59
5/4/2015 Closing Date [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Public offerings of common stock, Closing Date May 04, 2015
Public offerings of common stock, Shares Issued | shares 4,600
Public offerings of common stock, Price Per Share | $ / shares $ 18.50
Net proceeds from follow-on from public offering $ 82
10/19/2015 Closing Date [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Public offerings of common stock, Closing Date Oct. 19, 2015
Public offerings of common stock, Shares Issued | shares 5,750
Public offerings of common stock, Price Per Share | $ / shares $ 18.00
Net proceeds from follow-on from public offering $ 99