Debt Structure (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Unsecured notes payable	$ 514,000,000		$ 135,000,000
Total Outstanding Debt	1,706,258,000		1,649,755,000
Effective Rate	3.70%
Contract Maturity	Nov. 21, 2017
Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	1,192,258,000
Effective Rate	3.90%
Contract Maturity	May 30, 2017
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	514,000,000
Effective Rate	3.40%
Contract Maturity	Dec. 31, 2018
Fixed Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	680,000,000		538,000,000
Fixed Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	850,375,000
Effective Rate	5.00%
Contract Maturity	Sep. 10, 2016
Fixed Rate Debt | Secured Debt [Member] | Individual property mortgages
Debt Instrument [Line Items]
Total Outstanding Debt	373,575,000
Effective Rate	4.80%
Contract Maturity	Aug. 28, 2019
Fixed Rate Debt | Secured Debt [Member] | LIBOR-based interest rate swaps
Debt Instrument [Line Items]
Total Outstanding Debt	409,000,000
Effective Rate	5.30%
Contract Maturity	Feb. 03, 2014
Fixed Rate Debt | Secured Debt [Member] | SIFMA-based interest rate swaps
Debt Instrument [Line Items]
Total Outstanding Debt	17,800,000
Effective Rate	4.40%
Contract Maturity	Oct. 15, 2012
Fixed Rate Debt | Unsecured Debt [Member] | Senior Guaranteed Notes
Debt Instrument [Line Items]
Total Outstanding Debt	256,000,000
Effective Rate	4.60%
Contract Maturity	May 30, 2021
Fixed Rate Debt | Unsecured Debt [Member] | LIBOR-based interest rate swaps
Debt Instrument [Line Items]
Total Outstanding Debt	150,000,000	[1]
Effective Rate	2.70%
Contract Maturity	Mar. 01, 2017
Variable Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	1,027,000,000		1,112,000,000
Variable Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	341,883,000
Effective Rate	1.00%
Contract Maturity	Mar. 11, 2019
Variable Rate Debt | Unsecured Debt [Member] | Conventional Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	108,000,000
Effective Rate	1.30%
Contract Maturity	Nov. 01, 2015
FNMA | Fixed Rate Debt | Secured Debt [Member] | Conventional Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	50,000,000
Effective Rate	4.70%
Contract Maturity	Mar. 31, 2017
FNMA | Variable Rate Debt | Secured Debt [Member] | Conventional Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	189,721,000	[2]
Effective Rate	0.80%
Contract Maturity	Apr. 20, 2016
FNMA | Variable Rate Debt | Secured Debt [Member] | Tax Free Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	72,715,000	[2]
Effective Rate	1.00%
Contract Maturity	Jul. 23, 2031
Freddie Mac | Variable Rate Debt | Secured Debt [Member] | Conventional Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	64,247,000	[2]
Effective Rate	0.80%
Contract Maturity	Jul. 01, 2014
Freddie Mac | Variable Rate Debt | Secured Debt [Member] | Mortgages
Debt Instrument [Line Items]
Total Outstanding Debt	$ 15,200,000	[2]
Effective Rate	3.60%
Contract Maturity	Dec. 10, 2015

[1] 1) Includes capped balances.

[2] Includes capped balances.