Fair Value of Financial Instruments (Details) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loan receivable, fair value	$ 63,800,000		$ 45,000,000
Mortgage loan receivable, carrying value	64,687,000		45,279,000
Mortgage Notes Payable, fair value	494,200,000		545,600,000
The carrying amount notes, including notes associated with properties held for sale	446,472,000		470,687,000
Unsecured senior notes payable, fair value	748,600,000		725,900,000
Term Loan, fair value	248,100,000
Term loan	250,000,000
Redeemable noncontrolling interests	22,621,000		22,804,000
Investments in and advances to unconsolidated joint ventures	61,449,000	[1]	50,158,000	[1]
Assets Held For Sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
The carrying amount notes, including notes associated with properties held for sale	454,900,000		509,600,000
Unsecured Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unsecured senior notes payable, carrying value	679,300,000		688,800,000
Term Loan And Interest Rate Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 7,700,000

[1] With the exception of the Madison 2260 Realty LLC and Madison 1235 Realty LLC joint ventures, which are accounted for under the cost method, all unconsolidated joint ventures are accounted for under the equity method.