Supplemental Information to Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Supplemental schedule of noncash investing and financing activities:
Distribution payable on preferred units	$ 4,798	$ 452
Write-off of fully depreciated assets	(23,135)	(21,150)
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
Exchange of Limited partnership units for general partnership units:
Conversion of Limited Partner Units to General Partner Units
Limited Partner Units
Exchange of Limited partnership units for general partnership units:
Conversion of Limited Partner Units to General Partner Units	(2,469)	(833)
General Partner Units
Exchange of Limited partnership units for general partnership units:
Conversion of Limited Partner Units to General Partner Units	$ 2,469	$ 833