Supplemental Information to Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 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	5,214
Write-off of fully depreciated assets	(13,196)	(13,514)
Mortgage loan payable assumed in conjunction with a property acquisition	(12,026)
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	(99)
Common Stock [Member]
Exchange of Units for common stock
Conversion of Units to Common Stock
Additional Paid-in Capital [Member]
Exchange of Units for common stock
Conversion of Units to Common Stock	$ 99