Supplemental Information to Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Exchange of Units for common stock
Conversion of Units to Common Stock
Supplemental schedule of non-cash investing and financing activities
Distribution payable on preferred stock	4,798	452
Write-off of fully depreciated assets	(27,754)	(23,443)
Mortgage loan payable assumed in conjunction with a property acquisition	(12,026)	(24,417)
Notes receivable issued in conjunction with certain property sales		1,029
Noncontrolling Interest [Member]
Exchange of Units for common stock
Conversion of Units to Common Stock	(2,469)	(833)
Common Stock [Member]
Exchange of Units for common stock
Conversion of Units to Common Stock	3	1
Additional Paid-in Capital [Member]
Exchange of Units for common stock
Conversion of Units to Common Stock	$ 2,466	$ 832