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Notes Payable (Summary Of Indebtedness) (Details) (USD $)
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Debt Instrument [Line Items]    
Unsecured notes payable $ 1,380,755,000 $ 1,507,757,000
Total secured notes payable 1,051,357,000 1,055,997,000
Total notes payable 2,432,100,000 2,563,800,000
Commercial Banks [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable   500,000,000
Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 1,380,700,000 972,400,000
Term Loan, Due 2012 [Member] | Commercial Banks [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable   500,000,000
Notes payable, maturity date 2012  
7.69% Notes Due 2011 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable   88,000,000
Notes payable, maturity date 2012  
Effective annual interest rate on secured credit 7.69%  
5.93% Notes Due 2012 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 189,600,000 189,500,000
Notes payable, maturity date 2013  
Effective annual interest rate on secured credit 5.93%  
5.45% Notes Due 2013 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 199,700,000 199,600,000
Notes payable, maturity date 2015  
Effective annual interest rate on secured credit 5.45%  
5.08% Notes Due 2015 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 249,300,000 249,200,000
Notes payable, maturity date 2017  
Effective annual interest rate on secured credit 5.08%  
5.75% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 246,200,000 246,100,000
Effective annual interest rate on secured credit 5.75%  
4.70% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 248,600,000  
Notes payable, maturity date 2021  
Effective annual interest rate on secured credit 4.70%  
5.00% Notes Due 2023 [Member] | Senior Unsecured Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable 247,300,000  
Notes payable, maturity date 2023  
Effective annual interest rate on secured credit 5.00%  
4.99% Notes Due 2011 [Member] | Medium-Term Notes [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable   35,400,000
Notes payable, maturity date 2011  
Effective annual interest rate on secured credit 4.99%  
0.91% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member]
    
Debt Instrument [Line Items]    
Total secured notes payable 1,012,300,000 1,015,700,000
1.59% Tax-Exempt Mortgage Note, Due 2028 [Member]
    
Debt Instrument [Line Items]    
Total secured notes payable 39,100,000 40,300,000
Notes payable, maturity date 2028  
Effective annual interest rate on secured credit 1.59%  
Floating Rate Tax-Exempt Debt Inlcuded In Secured Notes (1.59%) [Member]
    
Debt Instrument [Line Items]    
Total secured notes payable 39,100,000 40,300,000
Effective annual interest rate on secured credit 1.59%  
Floating Rate Debt Included In Secured Notes (0.91% - 1.73%) [Member]
    
Debt Instrument [Line Items]    
Total secured notes payable 206,400,000 189,900,000
Value Of Real Estate Assets, At Cost [Member]
    
Debt Instrument [Line Items]    
Value of real estate assets, at cost, subject to secured notes 1,651,000,000 1,629,600,000
Total [Member]
    
Debt Instrument [Line Items]    
Unsecured notes payable $ 1,380,700,000 $ 1,507,800,000
Minimum [Member] | 0.91% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member]
    
Debt Instrument [Line Items]    
Notes payable, maturity date 2012  
Effective annual interest rate on secured credit 0.91%  
Minimum [Member] | Floating Rate Debt Included In Secured Notes (0.91% - 1.73%) [Member]
    
Debt Instrument [Line Items]    
Effective annual interest rate on secured credit 0.91%  
Maximum [Member] | 0.91% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member]
    
Debt Instrument [Line Items]    
Notes payable, maturity date 2045  
Effective annual interest rate on secured credit 6.00%  
Maximum [Member] | Floating Rate Debt Included In Secured Notes (0.91% - 1.73%) [Member]
    
Debt Instrument [Line Items]    
Effective annual interest rate on secured credit 1.73%