Quarterly report pursuant to Section 13 or 15(d)

Equity - Monthly Distributions Declared and Paid by Company's Board of Directors (Detail)

v3.8.0.1
Equity - Monthly Distributions Declared and Paid by Company's Board of Directors (Detail) - $ / shares
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Class of Stock [Line Items]    
Distributions per Common Share (in dollars per share) $ 0.3915 $ 0.3675
January 31, 2017    
Class of Stock [Line Items]    
Declaration Date Jan. 10, 2017  
Record Date Jan. 20, 2017  
Payment Date Jan. 31, 2017  
Distributions per Common Share (in dollars per share) $ 0.043  
February 28, 2017    
Class of Stock [Line Items]    
Declaration Date Jan. 10, 2017  
Record Date Feb. 16, 2017  
Payment Date Feb. 28, 2017  
Distributions per Common Share (in dollars per share) $ 0.043  
March 31, 2017    
Class of Stock [Line Items]    
Declaration Date Jan. 10, 2017  
Record Date Mar. 22, 2017  
Payment Date Mar. 31, 2017  
Distributions per Common Share (in dollars per share) $ 0.043  
April 28, 2017    
Class of Stock [Line Items]    
Declaration Date Apr. 11, 2017  
Record Date Apr. 21, 2017  
Payment Date Apr. 28, 2017  
Distributions per Common Share (in dollars per share) $ 0.04350  
May 31, 2017    
Class of Stock [Line Items]    
Declaration Date Apr. 11, 2017  
Record Date May 19, 2017  
Payment Date May 31, 2017  
Distributions per Common Share (in dollars per share) $ 0.0435  
June 30, 2017    
Class of Stock [Line Items]    
Declaration Date Apr. 11, 2017  
Record Date Jun. 21, 2017  
Payment Date Jun. 30, 2017  
Distributions per Common Share (in dollars per share) $ 0.0435  
10/31/2017    
Class of Stock [Line Items]    
Declaration Date Jul. 11, 2017  
Record Date Jul. 21, 2017  
Payment Date Jul. 31, 2017  
Distributions per Common Share (in dollars per share) $ 0.0440  
11/30/2017    
Class of Stock [Line Items]    
Declaration Date Jul. 11, 2017  
Record Date Aug. 21, 2017  
Payment Date Aug. 31, 2017  
Distributions per Common Share (in dollars per share) $ 0.0440  
12/29/2017    
Class of Stock [Line Items]    
Declaration Date Jul. 11, 2017  
Record Date Sep. 20, 2017  
Payment Date Sep. 29, 2017  
Distributions per Common Share (in dollars per share) $ 0.0440  
February 2, 2016    
Class of Stock [Line Items]    
Declaration Date   Jan. 12, 2016
Record Date   Jan. 22, 2016
Payment Date   Feb. 02, 2016
Distributions per Common Share (in dollars per share)   $ 0.040
February 29, 2016    
Class of Stock [Line Items]    
Declaration Date   Jan. 12, 2016
Record Date   Feb. 18, 2016
Payment Date   Feb. 29, 2016
Distributions per Common Share (in dollars per share)   $ 0.040
March 31, 2016    
Class of Stock [Line Items]    
Declaration Date   Jan. 12, 2016
Record Date   Mar. 21, 2016
Payment Date   Mar. 31, 2016
Distributions per Common Share (in dollars per share)   $ 0.040
May 2, 2016    
Class of Stock [Line Items]    
Declaration Date   Apr. 12, 2016
Record Date   Apr. 22, 2016
Payment Date   May 02, 2016
Distributions per Common Share (in dollars per share)   $ 0.04125
May 31, 2016    
Class of Stock [Line Items]    
Declaration Date   Apr. 12, 2016
Record Date   May 19, 2016
Payment Date   May 31, 2016
Distributions per Common Share (in dollars per share)   $ 0.04125
June 30, 2016    
Class of Stock [Line Items]    
Declaration Date   Apr. 12, 2016
Record Date   Jun. 17, 2016
Payment Date   Jun. 30, 2016
Distributions per Common Share (in dollars per share)   $ 0.04125
August 2, 2016    
Class of Stock [Line Items]    
Declaration Date   Jul. 12, 2016
Record Date   Jul. 22, 2016
Payment Date   Aug. 02, 2016
Distributions per Common Share (in dollars per share)   $ 0.04125
August 31, 2016    
Class of Stock [Line Items]    
Declaration Date   Jul. 12, 2016
Record Date   Aug. 22, 2016
Payment Date   Aug. 31, 2016
Distributions per Common Share (in dollars per share)   $ 0.04125
September 30, 2016    
Class of Stock [Line Items]    
Declaration Date   Jul. 12, 2016
Record Date   Sep. 21, 2016
Payment Date   Sep. 30, 2016
Distributions per Common Share (in dollars per share)   $ 0.04125