Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Company's estimates of fair value differ from the carrying amounts
Mortgage payable, net of premiums, Carrying Amount	$ 512,391		$ 712,246
Mortgage payable, net of premiums, Fair Value	545,784		726,348
Unsecured notes payable, net of discounts, Carrying Amount	1,496,501		1,277,716
Unsecured notes payable, net of discounts, Fair Value	1,555,633		1,338,743
Variable rate debt instruments, Carrying Amount	391,610		444,610
Variable rate debt instruments, Fair Value	380,786		432,556
Notes receivable, Carrying Amount	31,157	[1]	31,216	[1]
Notes receivable, Fair Value	$ 32,756		$ 28,921

[1] For purposes of this disclosure, one of the notes is presented gross of the deferred gain of $12.9 million arising from the sale of the two Trenton properties in 2009 accounted for under the accounting standard for installment sales.