Schedule III Part II - Summary of Real Estate and Accumulated Depreciation (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real Estate and Accumulated Depreciation, Carrying Amount of Land and Buildings and Improvements [Abstract]
Balance at beginning of year	$ 3,895,942,000	$ 3,759,234,000	$ 3,673,685,000
Acquisitions	439,061,000	34,855,000	34,485,000
Consolidation of VIE		18,311,000
Improvements	147,793,000	97,129,000	93,304,000
Deconsolidation of VIE	(18,311,000)
Deduction during period - disposition and retirements of property	(29,941,000)	(13,587,000)	(42,240,000)
Balance at end of year	4,434,544,000	3,895,942,000	3,759,234,000
Real Estate and Accumulated Depreciation, Other Required Disclosures [Abstract]
Real Estate, Federal Income Tax Basis	3,900,000,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year	1,035,204,000	938,087,000	846,258,000
Additions during period—depreciation and amortization expense	114,180,000	108,261,000	103,698,000
Deductions during period—disposition and retirements of property	(21,796,000)	(11,144,000)	(11,869,000)
Balance at end of year	$ 1,127,588,000	$ 1,035,204,000	$ 938,087,000