SEC Schedule III Rollforward of Real Estate Assets 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
Real estate at Cost	$ 17,387,701	$ 17,293,970	$ 17,140,726
Real Estate Other Acquisitions	33,481	347,345	0
Real Estate Improvements	315,762	324,114	601,136
Real Estate Cost Of Real Estate Sold	109,933	577,728	447,892
Real estate at Cost	17,627,011	17,387,701	17,293,970
Reconciliation Of Real Estate Accumulated Depreciation Roll Forward
Real Estate Investment Property, Accumulated Depreciation	2,715,046	2,395,608	2,068,357
Real Estate Accumulated Depreciation Depreciation Expense	452,793	428,788	433,785
Real Estate Accumulated Depreciation Real Estate Sold	72,802	109,350	106,534
Real Estate Investment Property, Accumulated Depreciation	$ 3,095,037	$ 2,715,046	$ 2,395,608