Annual report pursuant to Section 13 and 15(d)

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

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Real Estate Investments And Accumulated Depreciation (Schedule Of Changes In Rental Properties And Accumulated Depreciation) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Real Estate Investments And Accumulated Depreciation [Abstract]      
Rental Properties, Balance at beginning of year $ 5,279,770 $ 5,216,720 $ 5,186,208
Rental Properties, Additions 296,079 91,716 86,455
Rental Properties, Rental property held for sale (84,716)    
Rental Properties, Properties sold (34,563)   (16,052)
Rental Properties, Impairment charge on rental property (20,573)   (12,560)
Rental Properties, Retirements/disposals (56,561) (28,666) (27,331)
Rental Properties, Balance at end of year 5,379,436 5,279,770 5,216,720
Accumulated Depreciation, Balance at beginning of year 1,409,163 1,278,985 1,153,223
Accumulated Depreciation, Depreciation expense 157,175 158,559 158,318
Accumulated Depreciation, Rental property held for sale (23,852)    
Accumulated Depreciation, Properties sold (10,026)   (2,091)
Accumulated Depreciation, Impairment charge on rental property 2,058   (3,256)
Accumulated Depreciation, Retirements/disposals (56,304) (28,381) (27,209)
Accumulated Depreciation, Balance at end of year $ 1,478,214 $ 1,409,163 $ 1,278,985