Annual report pursuant to Section 13 and 15(d)

Real Estate Investments And Accumulated Depreciation (Changes In Rental Properties And Accumulated Depreciation) (Details)

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Real Estate Investments And Accumulated Depreciation (Changes In Rental Properties And Accumulated Depreciation) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Real Estate Investments And Accumulated Depreciation [Abstract]      
Rental Properties, Balance at beginning of year $ 5,216,720 $ 5,186,208 $ 4,963,780
Rental Properties, Additions 91,716 86,455 282,122
Rental Properties, Properties sold   (16,052)  
Rental Properties, Impairment charge on rental property   (12,560)  
Rental Properties, Retirements/disposals (28,666) (27,331) (59,694)
Rental Properties, Balance at end of year 5,279,770 5,216,720 5,186,208
Accumulated Depreciation, Balance at beginning of year 1,278,985 1,153,223 1,040,778
Accumulated Depreciation, Depreciation expense 158,559 158,318 155,613
Accumulated Depreciation, Properties sold   (2,091)  
Accumulated Depreciation, Impairment charge on rental property   (3,256) (1,009)
Accumulated Depreciation, Retirements/disposals (28,381) (27,209) (42,159)
Accumulated Depreciation, Balance at end of year $ 1,409,163 $ 1,278,985 $ 1,153,223