Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Company's estimates of fair value differ from the carrying amounts
Mortgage notes payable, Carrying Amount	$ 443,942		$ 512,391
Mortgage notes payable, Fair Value	486,412		545,784
Unsecured notes payable, Carrying Amount	1,430,343		1,496,501
Unsecured notes payable, Fair Value	1,553,123		1,555,633
Variable rate debt instruments, Carrying Amount	597,610		391,610
Variable rate debt instruments, Fair Value	595,693		380,786
Notes receivable, Carrying Amount	7,226	[1]	31,157	[1]
Notes receivable, Fair Value	$ 7,783		$ 32,756

[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 two properties in 2009 accounted for under the accounting standard for installment sales.