Unsecured Line Of Credit And Term Notes - Additional Information (Detail) (USD $)	9 Months Ended		0 Months Ended		0 Months Ended	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Aug. 05, 2011 Maturity In August 2018 [Member]	Sep. 30, 2012 Maturity In August 2018 [Member]	Aug. 05, 2011 Maturity In August 2021 [Member]	Sep. 30, 2012 Maturity In August 2021 [Member]	Sep. 30, 2012 Maturity In September 2013 [Member]	Sep. 30, 2012 Maturity In September 2013 [Member] Fixed Interest Rate [Member]	Sep. 30, 2012 Maturity In April 2016 [Member]	Sep. 30, 2012 Maturity In August 2016 [Member] MortgageLoan	Dec. 31, 2011 Maturity In August 2016 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Unsecured term note	$ 575,000,000	$ 575,000,000	$ 125,000,000		$ 100,000,000		$ 20,000,000	$ 80,000,000	$ 150,000,000
Basis spread over LIBOR				2.00%			1.50%			2.00%
Revolving line of credit facility agreement	175,000,000
Revolving line of credit facility expandable amount	250,000,000
Number of one year extensions										2
Facility fee										0.20%
Amount available on unsecured line of credit										149,000,000
Extension facility fee	0.125%
Interest rate, line of credit facility										2.22%	2.28%
Line of credit facility, expiration date			Aug. 03, 2018		Aug. 05, 2021		Sep. 04, 2013	Sep. 04, 2013	Apr. 13, 2016	Aug. 04, 2016
Additional secured term note			$ 100,000,000
Line of credit facility, maturity date range, start			Dec. 01, 2011
Term note stated interest rate					5.54%			6.26%	6.38%
Increase in interest rate on term loan						7.29%			8.13%