Mortgage Notes Payable (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Mortgages notes payable	$ 1,565,599,000		$ 1,745,858,000
Number of properties securing mortgage notes	55		68
Weighted average interest rate	5.40%		5.40%
Monthly interest expense	7,000,000
Monthly principal amortization	2,000,000
Mortgages notes payable	1,565,599,000		1,745,858,000
Repayment of debt before the scheduled maturity date, prepayment penalties	The prepayment penalty on the majority of the Company's mortgage notes payable are computed by the greater of (a) 1% of the amount of the principal being prepaid or (b) the present value of the mortgage note payable which is calculated by multiplying the principal being prepaid by the difference between the interest rate of the mortgage note and the stated yield rate on a specified U.S. treasury security as defined in the mortgage note agreement.
Mortgage notes payable maturities
2013	57,621,000
2014	47,994,000
2015	68,926,000
2016	12,656,000
2017	185,301,000
Thereafter	1,193,101,000
Minimum [Member]
Debt Instrument [Line Items]
Remaining terms	P1Y		P1Y
Maximum [Member]
Debt Instrument [Line Items]
Remaining terms	P27Y		P28Y
Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Mortgages notes payable	1,363,731,000		1,502,208,000
Multifamily Housing Mortgage Revenue Bonds [Member]
Debt Instrument [Line Items]
Mortgages notes payable	201,868,000	[1]	243,650,000	[1]
Multifamily housing mortgage revenue bonds, variable interest rate (in hundredths)	1.90%		2.00%
Multifamily housing mortgage revenue bonds credit enhancement and underwriting fees, minimum (in hundredths)	1.20%
Multifamily housing mortgage revenue bonds credit enhancement and underwriting fees, maximum (in hundredths)	1.90%
Percentage of units subject to tenant income criteria (in hundredths)	20.00%
Bond subject to interest rate caps	187,800,000
Secured Deeds of Trust [Member]
Debt Instrument [Line Items]
Mortgages notes payable	$ 87,300,000

[1] Variable rate mortgage notes payable consists of multifamily housing mortgage revenue bonds secured by deeds of trust on rental properties and guaranteed by collateral pledge agreements, payable monthly at a variable rate as defined in the Loan Agreement (approximately 1.9% at December 2012 and 2.0% at December 2011) plus credit enhancement and underwriting fees ranging from approximately 1.2% to 1.9%. Among the terms imposed on the properties, which are security for the bonds, is a requirement that 20% of the units are subject to tenant income criteria. Principal balances are due in full at various maturity dates from October 2013 through December 2039. Of these bonds $187.8 million are subject to various interest rate cap agreements which limit the maximum interest rate to such bonds.