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Indebtedness (Details) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended 6 Months Ended 1 Months Ended 6 Months Ended 1 Months Ended
Jun. 30, 2012
item
Jun. 30, 2011
Dec. 31, 2011
Jun. 30, 2012
Unsecured revolving credit facility
Jun. 30, 2012
Senior unsecured notes 4.30%
Jun. 30, 2012
Senior unsecured notes 6.75%
Jun. 30, 2012
Senior unsecured notes 6.75% maturing in 2021
Jun. 30, 2012
Mortgages
item
Apr. 30, 2012
Mortgages
item
Feb. 29, 2012
Mortgages
item
Jun. 30, 2012
Mortgages
item
Jun. 30, 2012
Mortgages
Significant Other Observable Inputs (Level 2)
Jun. 30, 2012
Capital leases
item
Jan. 31, 2012
Senior unsecured notes 8.625%
Jul. 31, 2012
Senior unsecured notes 5.625% due in 2042
Indebtedness                              
Unsecured revolving credit facility, maximum borrowing capacity       $ 750,000                      
Number of public issues of unsecured senior notes 3                            
Principal amount of unsecured senior notes 741,412   965,770   250,000 200,000 300,000                
Aggregate principal amount of mortgage debt               849,511     849,511        
Number of properties mortgaged               63 17   63        
Carrying value of mortgaged properties 4,177,983   4,091,330         1,100,704     1,100,704   15,760    
Mortgage debt assumed in connection with real estate acquisitions 56,789 48,062                   56,789      
Weighted average maturity period of mortgage debt assumed                     5 years 10 months 24 days        
Repayments of borrowings under revolving credit facility                           225,000  
Number of properties secured by mortgage loans               1   1          
Number of properties recorded under capital lease                         2    
Capital leases                         14,005    
Sale of senior unsecured notes                             350,000
Net proceeds from sale of senior unsecured notes                             338,800
Interest rate, description       LIBOR                      
Interest rate added to the base rate (as a percent)       1.60%                      
Revolving credit facility, interest rate payable (as a percent)       1.80%                      
Unsecured revolving credit facility, amount outstanding       360,000                      
Revolving credit facility, available amount       390,000                      
Number of mortgage loans                 17            
Repayment of mortgage loans               3,140 32,576 12,400          
Interest rate (as a percent)         4.30% 6.75% 6.75% 6.07%   6.03% 6.07%     8.625% 5.625%
Weighted average interest rate on mortgage debt assumed (as a percent)               5.54% 6.95%   5.54%        
Extension period of revolving credit facility       1 year                      
Option to increase the borrowing capacity under revolving credit facility       $ 1,500,000