Notes Payable - Additional Information (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Fixed Rate Debt	Dec. 31, 2011 Fixed Rate Debt	Mar. 01, 2012 Unsecured Debt [Member] $150 million unsecured term loan [Domain]	Sep. 30, 2012 Unsecured Debt [Member] Senior Guaranteed Notes	Sep. 30, 2012 Interest Rate Swaps	Sep. 30, 2012 Interest Rate Caps	Sep. 30, 2012 Not Designated as Hedging Instrument [Member]	Sep. 30, 2012 Secured Debt [Member]	Sep. 30, 2012 Secured Debt [Member] Fixed Rate Debt	Sep. 30, 2012 Secured Debt [Member] Individual Property Mortgages [Member] Fixed Rate Debt	Sep. 30, 2012 Secured Debt [Member] L I B O R [Member] Fixed Rate Debt	Sep. 30, 2012 Unsecured Debt [Member]	Sep. 30, 2012 Unsecured Debt [Member] L I B O R [Member] Fixed Rate Debt		Sep. 30, 2012 Unsecured Debt [Member] Senior Guaranteed Notes Fixed Rate Debt	Sep. 30, 2012 Minimum $150 million unsecured term loan [Domain]	Mar. 01, 2012 Minimum $150 million unsecured term loan [Domain]	Sep. 30, 2012 Maximum $150 million unsecured term loan [Domain]	Mar. 01, 2012 Maximum $150 million unsecured term loan [Domain]	Sep. 30, 2012 Tranche 1 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Sep. 30, 2012 Tranche 2 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Sep. 30, 2012 Tranche 3 [Member] Unsecured Debt [Member] Senior Guaranteed Notes	Sep. 30, 2012 Tranche 4 [Member] Unsecured Debt [Member] Senior Guaranteed Notes
Debt Instrument [Line Items]
Notes Payable	$ 1,706,258,000	$ 1,649,755,000	$ 680,000,000	$ 538,000,000		$ 121,000,000				$ 1,192,258,000	$ 850,375,000	$ 373,575,000	$ 409,000,000	$ 514,000,000	$ 150,000,000	[1]	$ 256,000,000
Debt Instrument, Face Amount					150,000,000	175,000,000																18,000,000	20,000,000	117,000,000	20,000,000
Debt Instrument, Interest Rate, Stated Percentage																						3.15%	3.61%	4.17%	4.33%
Debt Instrument, Basis Spread on Variable Rate																		1.10%	1.40%	2.05%	2.15%
Secured credit facility as a Percentage of outstanding debt	47.00%
Number of interest rate contracts							22	18
Notional							$ 576,800,000	$ 256,400,000	$ 55,875,000
Derivative, Number of Instruments Held									9
Debt Instrument, Interest Rate, Effective Percentage	3.70%									3.90%	5.00%	4.80%	5.30%	3.40%	2.70%		4.60%

[1] 1) Includes capped balances.