Schedule III-Real Estate And Accumulated Depreciation (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in real estate
Balance at beginning of year	$ 11,147,151,000	$ 8,890,064,000	$ 8,257,100,000
Property acquisitions	649,445,000	2,276,308,000	703,721,000
Improvements	146,410,000	162,875,000	97,099,000
Retirements/disposals/deconsolidation	(280,053,000)	(182,096,000)	(167,856,000)
Balance at end of year	11,662,953,000	11,147,151,000	8,890,064,000
Aggregate cost of land, buildings and improvements, before depreciation, for Federal income tax purposes	8,300,000,000	7,800,000,000
Changes in accumulated depreciation, exclusive of amounts relating to equipment, autos, and furniture and fixtures
Balance at beginning of year	1,136,603,000	916,293,000	738,422,000
Depreciation for year	288,560,000	245,421,000	209,472,000
Retirements/disposals/deconsolidation	(31,840,000)	(25,111,000)	(31,601,000)
Balance at end of year	$ 1,393,323,000	$ 1,136,603,000	$ 916,293,000