Debt Structure (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Unsecured notes payable	$ 187,000,000		$ 135,000,000
Total Outstanding Debt	1,540,056,000		1,649,755,000
Effective Rate	3.80%
Contract Maturity	Jun. 02, 2017
Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	1,353,056,000
Effective Rate	3.70%
Contract Maturity	Feb. 09, 2017
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	187,000,000
Effective Rate	4.30%
Contract Maturity	Aug. 30, 2019
Fixed Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	537,000,000		538,000,000
Fixed Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	903,911,000
Effective Rate	5.10%
Contract Maturity	Aug. 07, 2016
Fixed Rate Debt | Secured Debt [Member] | Individual property mortgages
Debt Instrument [Line Items]
Total Outstanding Debt	352,111,000
Effective Rate	5.00%
Contract Maturity	Jul. 06, 2020
Fixed Rate Debt | Secured Debt [Member] | LIBOR-based interest rate swaps
Debt Instrument [Line Items]
Total Outstanding Debt	484,000,000
Effective Rate	5.30%
Contract Maturity	Oct. 30, 2013
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] | Interest Rate Swaps
Debt Instrument [Line Items]
Effective Rate	0.00%
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	50,000,000	[1]
Effective Rate	1.90%
Contract Maturity	Mar. 01, 2017
Variable Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	1,003,000,000		1,112,000,000
Variable Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	449,145,000
Effective Rate	0.80%
Contract Maturity	Feb. 18, 2018
Variable Rate Debt | Unsecured Debt [Member] | Conventional Credit Facility
Debt Instrument [Line Items]
Total Outstanding Debt	2,000,000
Effective Rate	2.70%
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	296,983,000	[2]
Effective Rate	0.70%
Contract Maturity	Oct. 01, 2015
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	0.70%
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] Balance is fixed at 2.7% by interest rate swaps that do not become effective until June 1, 2012. The unswapped rate has been used in this table.

[2] Includes capped balances.