Notes Payable, Credit Facility and Repurchase Agreements (Details) (USD $)
6 Months Ended 6 Months Ended 6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2012
Fixed rate debt
Jun. 30, 2012
Fixed rate debt
Minimum
Jun. 30, 2012
Fixed rate debt
Maximum
Jun. 30, 2012
Variable rate debt
Jun. 30, 2012
Variable rate debt
LIBOR
Jun. 30, 2012
Credit facility
Jun. 30, 2012
Credit facility
Term loan
Jun. 30, 2012
Credit facility
Term loan
LIBOR
Minimum
Jun. 30, 2012
Credit facility
Term loan
LIBOR
Maximum
Jun. 30, 2012
Credit facility
Revolving credit facility
Jun. 30, 2012
Credit facility
Letter of credit
Feb. 24, 2011
Credit facility
Fixed rate debt, variable rate debt fixed through the use of interest rate swaps
Term loan
LIBOR
Jun. 30, 2012
Credit facility
Company leverage ratio
Term loan
Jun. 30, 2012
Repurchase agreements
Debt Instrument [Line Items]                                
Notes payable and line of credit $ 1,747,505,000 $ 1,767,591,000 $ 1,400,000,000     $ 4,300,000   $ 316,100,000                
Gross real estate and related assets securing debt 2,500,000,000             635,400,000                
Debt, weighted average interest rate     5.85%         5.56%       3.93%        
Debt instrument, weighted average years to maturity               3 years 9 months 25 days                
Fixed rate debt, interest rate       3.52% 7.22%                      
Basis spread             2.75%     2.75% 4.00%       3.50%  
Debt repayments     44,000,000                         65,000,000
Debt borrowings                               10,700,000
Senior unsecured credit facility [Abstract]                                
Senior unsecured credit facility, current borrowing capacity               350,000,000 111,100,000     238,900,000 476,000      
Senior unsecured credit facility, additional borrowings               77,000,000                
Senior unsecured credit facility, repayments               54,000,000                
Senior unsecured credit facility, outstanding                 $ 111,100,000     $ 205,000,000        
Fixed interest rate through derivatives                           1.44%