Debt - Summary of Debt (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Weighted-average stated interest rate	3.33%	3.39%
Amount outstanding	$ 1,945,897	$ 1,876,631
Gross book value of properties encumbered by debt	2,404,595	2,285,998
Secured Line of Credit [Member]
Debt Instrument [Line Items]
Weighted-average stated interest rate	2.05%	2.32%
Maturity Date of Debt	Jan. 01, 2017
Amount outstanding	85,000	85,000
Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Weighted-average stated interest rate	1.90%	2.16%
Maturity Date of Debt	Aug. 01, 2015
Amount outstanding	223,000	165,000
Unsecured Term Loans [Member]
Debt Instrument [Line Items]
Weighted-average stated interest rate	2.13%	2.03%
Maturity Date of Debt, Earliest	Jan. 01, 2018
Maturity Date of Debt, Latest	Jan. 01, 2019
Amount outstanding	500,000	500,000
Variable-Rate Mortgage Note [Member]
Debt Instrument [Line Items]
Weighted-average stated interest rate	2.15%	2.19%
Maturity Date of Debt	May 01, 2015
Amount outstanding	9,080	9,080
Fixed-Rate Mortgage Notes [Member]
Debt Instrument [Line Items]
Weighted-average stated interest rate	4.25%	4.26%
Maturity Date of Debt, Earliest	Jun. 01, 2015
Maturity Date of Debt, Latest	Nov. 01, 2024
Amount outstanding	$ 1,128,817	$ 1,117,551