Debt Structure (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Unsecured notes payable	$ 577,000,000		$ 483,000,000
Total Outstanding Debt	1,691,253,000		1,673,848,000
Effective Rate	3.60%
Contract Maturity	Jun. 15, 2018
Unsecured Debt [Member]
Debt Instrument [Line Items]
Effective Rate	3.40%
Contract Maturity	Apr. 06, 2019
Secured Debt [Member]
Debt Instrument [Line Items]
Effective Rate	3.70%	[1]
Contract Maturity	Jan. 14, 2018	[1]
Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	1,114,253,000	[1]
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	577,000,000
Fixed Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Effective Rate	5.00%
Contract Maturity	Feb. 19, 2017
Fixed Rate Debt | Term Fixed Loan With Swaps [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	150,000,000
Effective Rate	2.40%
Contract Maturity	Mar. 01, 2017
Fixed Rate Debt | Senior Notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	310,000,000
Effective Rate	4.50%
Contract Maturity	Jul. 27, 2021
Fixed Rate Debt | L I B O R [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	334,000,000
Effective Rate	5.30%
Contract Maturity	Apr. 22, 2014
Fixed Rate Debt | Individual Property Mortgages [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	371,116,000
Effective Rate	4.80%
Contract Maturity	Sep. 02, 2019
Fixed Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	755,116,000
Variable Rate Debt | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	117,000,000
Effective Rate	1.50%
Contract Maturity	Nov. 01, 2015
Variable Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Effective Rate	0.90%	[1]
Contract Maturity	Dec. 06, 2019	[1]
Variable Rate Debt | Mortgages [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	15,200,000	[1]
Effective Rate	3.60%	[1]
Variable Rate Debt | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	359,137,000	[1]
FNMA | Fixed Rate Debt | Conventional Credit Facility [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	50,000,000
Effective Rate	4.70%
Contract Maturity	Mar. 31, 2017
FNMA | Variable Rate Debt | Conventional Credit Facility [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	189,721,000	[1]
Effective Rate	0.80%	[1]
Contract Maturity	Jul. 27, 2016	[1]
FNMA | Variable Rate Debt | Tax Free Credit Facility [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	89,969,000	[1]
Effective Rate	0.90%	[1]
Contract Maturity	Jul. 23, 2031	[1]
Freddie Mac | Variable Rate Debt | Conventional Credit Facility [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	$ 64,247,000	[1]
Effective Rate	0.70%	[1]
Contract Maturity	Jul. 01, 2014	[1]
Freddie Mac | Variable Rate Debt | Mortgages [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Contract Maturity	Jan. 01, 2016	[1]

[1] Includes capped balances.