Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011 Property Entity
Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of apartment properties owned by unconsolidated variable interest entities	268
Number of variable interest entities unconsolidated	215
Number of units in apartment properties owned by unconsolidated variable interest entities	15,818
General partner or equivalent interest in consolidated tax credit funds that are VIEs and that own an interest in unconsolidated VIEs	1% or less
Investments in unconsolidated VIEs through consolidated VIEs in which we hold an insignificant economic interest	$ 33.4
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities consolidated	124
Number of apartment properties owned by consolidated variable interest entities	84
Number of units in apartment properties owned by consolidated variable interest entities	12,982
Carrying value of real estate collateralizing debt of consolidated VIEs	805.4
Carrying value of property debt of consolidated VIEs	641.8
Obligation to Advance Funds to Unconsolidated Variable Interest Entities [Member]
Basis Of Presentation [Line Items]
Maximum risk of loss related to investments in, receivables from, or contractual obligation to advance funds to unconsolidated VIEs	3.2
Variable Interest Entity, Not Primary Beneficiary, Held Through Consolidated Tax Credit Funds in Which Company Holds Substantially All Economic Interests [Member]
Basis Of Presentation [Line Items]
Investments in unconsolidated VIEs held through consolidated tax credit funds that are VIEs and in which we hold substantially all the economic interests	5.6
Maximum risk of loss related to investments in, receivables from, or contractual obligation to advance funds to unconsolidated VIEs	5.6
Receivables from Unconsolidated Variable Interest Entities [Member]
Basis Of Presentation [Line Items]
Maximum risk of loss related to investments in, receivables from, or contractual obligation to advance funds to unconsolidated VIEs	$ 99.7