Rental Properties (Summary Of Carrying Amounts Of Rental Properties) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Real Estate Properties [Line Items]
Carrying amounts of rental properties	$ 2,178,516,000		$ 2,154,292,000
Accumulated depreciation	(347,586,000)		(335,116,000)
Total	1,830,930,000		1,819,176,000
Depreciation expense on rental properties	11,800,000	11,100,000
Building and improvements [Member]
Real Estate Properties [Line Items]
Carrying amounts of rental properties	1,630,071,000		1,602,676,000
Furniture, fixtures & equipment [Member]
Real Estate Properties [Line Items]
Carrying amounts of rental properties	50,976,000		54,737,000
Land [Member]
Real Estate Properties [Line Items]
Carrying amounts of rental properties	$ 497,469,000		$ 496,879,000