Schedule III - Real Estate and Accumulated Depreciation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in cost of properties
Beginning balance	$ 4,038,932	$ 3,948,487	$ 3,452,512
Acquisitions of operating properties	33,684	26,887	187,052
Building and land improvements	214,418	304,079	338,358
Sales	(291,491)	(75,315)	(29,430)
Impairments	(121,557)	(165,206)	0
Other dispositions	(13,891)	0	0
Other	(135)	0	(5)
Ending balance	3,859,960	4,038,932	3,948,487
Changes in accumulated depreciation
Beginning balance	577,601	503,032	422,612
Depreciation expense	93,158	99,173	88,048
Sales	(40,346)	(9,640)	(7,764)
Impairments	(58,855)	(15,039)	0
Other dispositions	(3,247)	0	0
Other	(135)	75	136
Ending balance	$ 568,176	$ 577,601	$ 503,032