Debt Structure (Detail) (USD $)	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Unsecured notes payable	$ 363,000,000		$ 135,000,000
Total Outstanding Debt	1,589,421,000		1,649,755,000
Effective Rate	3.80%
Contract Maturity	Jun. 28, 2017
Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	1,226,421,000
Effective Rate	3.90%
Contract Maturity	Apr. 15, 2017
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	363,000,000
Effective Rate	3.40%
Contract Maturity	Mar. 04, 2018
Fixed Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	569,000,000		538,000,000
Fixed Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	876,538,000
Effective Rate	5.00%
Contract Maturity	Jul. 29, 2016
Fixed Rate Debt | Secured Debt [Member] | Individual property mortgages
Debt Instrument [Line Items]
Total Outstanding Debt	374,738,000
Effective Rate	4.80%
Contract Maturity	Aug. 26, 2019
Fixed Rate Debt | Secured Debt [Member] | LIBOR-based interest rate swaps
Debt Instrument [Line Items]
Total Outstanding Debt	434,000,000
Effective Rate	5.20%
Contract Maturity	Jan. 01, 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	135,000,000
Effective Rate	5.10%
Contract Maturity	Aug. 23, 2020
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	1,030,000,000		1,112,000,000
Variable Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	349,883,000
Effective Rate	1.00%
Contract Maturity	Jan. 27, 2019
Variable Rate Debt | Unsecured Debt [Member] | Conventional Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	78,000,000
Effective Rate	1.70%
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	197,721,000	[2]
Effective Rate	0.80%
Contract Maturity	Mar. 18, 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) Excludes borrowings on properties categorized as Assets Held for Sale on the Consolidated Balance Sheet. As of June 30, 2012 held for sale properties include a $9.1 million mortgage with a fixed interest rate of 5.9% maturing on January 1, 2044.

[2] Includes capped balances.