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 |