Annual report pursuant to Section 13 and 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
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Class of Stock [Line Items]      
Distributions per common share (in dollars per share) $ 0.5238 $ 0.495 $ 0.465
Monthly Distribution One      
Class of Stock [Line Items]      
Declaration Date Jan. 10, 2017 Jan. 12, 2016 Jan. 13, 2015
Record Date Jan. 20, 2017 Jan. 22, 2016 Jan. 23, 2015
Payment Date Jan. 31, 2017 Feb. 02, 2016 Feb. 03, 2015
Distributions per common share (in dollars per share) $ 0.043 $ 0.040 $ 0.035
Monthly Distribution Two      
Class of Stock [Line Items]      
Declaration Date Jan. 10, 2017 Jan. 12, 2016 Jan. 13, 2015
Record Date Feb. 16, 2017 Feb. 18, 2016 Feb. 18, 2015
Payment Date Feb. 28, 2017 Feb. 29, 2016 Feb. 27, 2015
Distributions per common share (in dollars per share) $ 0.043 $ 0.040 $ 0.035
Monthly Distribution Three      
Class of Stock [Line Items]      
Declaration Date Jan. 10, 2017 Jan. 12, 2016 Jan. 13, 2015
Record Date Mar. 22, 2017 Mar. 21, 2016 Mar. 20, 2015
Payment Date Mar. 31, 2017 Mar. 31, 2016 Mar. 31, 2015
Distributions per common share (in dollars per share) $ 0.043 $ 0.040 $ 0.035
Monthly Distribution Four      
Class of Stock [Line Items]      
Declaration Date Apr. 11, 2017 Apr. 12, 2016 Apr. 14, 2015
Record Date Apr. 21, 2017 Apr. 22, 2016 Apr. 24, 2015
Payment Date Apr. 28, 2017 May 02, 2016 May 04, 2015
Distributions per common share (in dollars per share) $ 0.04350 $ 0.04125 $ 0.040
Monthly Distribution Five      
Class of Stock [Line Items]      
Declaration Date Apr. 11, 2017 Apr. 12, 2016 Apr. 14, 2015
Record Date May 19, 2017 May 19, 2016 May 19, 2015
Payment Date May 31, 2017 May 31, 2016 May 28, 2015
Distributions per common share (in dollars per share) $ 0.0435 $ 0.04125 $ 0.040
Monthly Distribution Six      
Class of Stock [Line Items]      
Declaration Date Apr. 11, 2017 Apr. 12, 2016 Apr. 14, 2015
Record Date Jun. 21, 2017 Jun. 17, 2016 Jun. 19, 2015
Payment Date Jun. 30, 2017 Jun. 30, 2016 Jun. 30, 2015
Distributions per common share (in dollars per share) $ 0.0435 $ 0.04125 $ 0.040
Monthly Distribution Seven      
Class of Stock [Line Items]      
Declaration Date Jul. 11, 2017 Jul. 12, 2016 Jul. 14, 2015
Record Date Jul. 21, 2017 Jul. 22, 2016 Jul. 24, 2015
Payment Date Jul. 31, 2017 Aug. 02, 2016 Aug. 04, 2015
Distributions per common share (in dollars per share) $ 0.0440 $ 0.04125 $ 0.040
Monthly Distribution Eight      
Class of Stock [Line Items]      
Declaration Date Jul. 11, 2017 Jul. 12, 2016 Jul. 14, 2015
Record Date Aug. 21, 2017 Aug. 22, 2016 Aug. 20, 2015
Payment Date Aug. 31, 2017 Aug. 31, 2016 Aug. 31, 2015
Distributions per common share (in dollars per share) $ 0.0440 $ 0.04125 $ 0.040
Monthly Distribution Nine      
Class of Stock [Line Items]      
Declaration Date Jul. 11, 2017 Jul. 12, 2016 Jul. 14, 2015
Record Date Sep. 20, 2017 Sep. 21, 2016 Sep. 21, 2015
Payment Date Sep. 29, 2017 Sep. 30, 2016 Sep. 30, 2015
Distributions per common share (in dollars per share) $ 0.0440 $ 0.04125 $ 0.040
Monthly Distribution Ten      
Class of Stock [Line Items]      
Declaration Date Oct. 10, 2017 Oct. 11, 2016 Oct. 13, 2015
Record Date Oct. 20, 2017 Oct. 21, 2016 Oct. 26, 2015
Payment Date Oct. 31, 2017 Oct. 31, 2016 Oct. 29, 2015
Distributions per common share (in dollars per share) $ 0.04410 $ 0.04250 $ 0.040
Monthly Distribution Eleven      
Class of Stock [Line Items]      
Declaration Date Oct. 10, 2017 Oct. 11, 2016 Oct. 13, 2015
Record Date Nov. 20, 2017 Nov. 17, 2016 Nov. 17, 2015
Payment Date Nov. 30, 2017 Nov. 30, 2016 Nov. 24, 2015
Distributions per common share (in dollars per share) $ 0.04410 $ 0.04250 $ 0.040
Monthly Distribution Twelve      
Class of Stock [Line Items]      
Declaration Date Oct. 10, 2017 Oct. 11, 2016 Oct. 13, 2015
Record Date Dec. 29, 2017 Dec. 20, 2016 Dec. 18, 2015
Payment Date Dec. 29, 2017 Dec. 30, 2016 Dec. 31, 2015
Distributions per common share (in dollars per share) $ 0.04410 $ 0.04250 $ 0.040