Secured Debt (Details) (USD $)	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Secured debt instruments
Principal Outstanding	$ 1,992,401,000	$ 1,908,068,000
Weighted Average Interest Rate	4.22%
Weighted Average Years to Maturity	5.1
Number of Communities Encumbered	66
Mortgage notes payable [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Principal Outstanding	564,129,000
Weighted Average Interest Rate	4.95%
Weighted Average Years to Maturity	4.0
Number of Communities Encumbered	10
Mortgage notes payable [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal Outstanding	177,201,000
Weighted Average Interest Rate	1.94%
Weighted Average Years to Maturity	2.6
Number of Communities Encumbered	8
Tax Exempt Notes Payable [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Principal Outstanding	0
Tax Exempt Notes Payable [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal Outstanding	94,700,000
Weighted Average Interest Rate	0.83%
Weighted Average Years to Maturity	11.1
Number of Communities Encumbered	2
Fannie Mae credit facilities [Member]
Secured debt instruments
Weighted Average Interest Rate	1.61%
Fannie Mae credit facilities [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Principal Outstanding	895,920,000
Weighted Average Interest Rate	5.32%
Weighted Average Years to Maturity	5.9
Number of Communities Encumbered	14
Fannie Mae credit facilities [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal Outstanding	260,451,000
Weighted Average Interest Rate	1.61%
Weighted Average Years to Maturity	4.6
Number of Communities Encumbered	32
Fixed Rate Debt [Member]
Secured debt instruments
Principal Outstanding	1,460,049,000
Weighted Average Interest Rate	5.18%
Weighted Average Years to Maturity	5.2
Number of Communities Encumbered	24
Variable Rate Debt [Member]
Secured debt instruments
Principal Outstanding	532,352,000,000
Weighted Average Interest Rate	1.58%
Weighted Average Years to Maturity	5.1
Number of Communities Encumbered	$ 42