Schedule III - Real Estate and Accumulated Depreciation (Reconciliation of Real Estate and Accumulated Depreciation) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at beginning of year	$ 4,834,111	$ 4,512,618	$ 4,608,320
Additions:
Acquisitions	31,454	102,475	0
Capital expenditures	133,550	336,281	80,506
Less:
Dispositions	(206,035)	(117,263)	(176,208)
Balance at end of year	4,793,080	4,834,111	4,512,618
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year	776,078	716,957	639,688
Additions:
Depreciation expense — continuing operations	157,691	133,740	141,309
Depreciation expense — discontinued operations	1,185	1,554	6,494
Less:
Dispositions	(69,244)	(76,173)	(70,534)
Balance at end of year	$ 865,710	$ 776,078	$ 716,957