Quarterly report pursuant to Section 13 or 15(d)

Debt, Net (Details)

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Debt, Net (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Debt          
Carrying Value $ 2,094,486,000   $ 2,094,486,000   $ 2,077,752,000
Deferred financing costs, net 4,497,000   4,497,000   5,867,000
Interest costs capitalized 1,300,000 $ 2,000,000 3,100,000 $ 4,100,000  
Loans payable          
Debt          
Deferred financing costs, net 7,400,000   7,400,000   8,000,000
Mortgage and Other Secured Loans:          
Debt          
Carrying Value 331,851,000   331,851,000   331,000,000
Fixed rate mortgage loans          
Debt          
Carrying Value 318,243,000   $ 318,243,000   281,208,000
Stated interest rates, low end of range (as a percent)     3.82%    
Stated interest rates, high end of range (as a percent)     7.87%    
Unamortized premium included in carrying value $ 459,000   $ 459,000   514,000
Weighted average interest rate (as a percent) 5.75%   5.75%    
Variable rate secured loans          
Debt          
Carrying Value $ 13,608,000   $ 13,608,000   49,792,000
Description of variable rate basis     LIBOR    
Stated interest rate (as a percent) 2.31%   2.31%    
Variable rate secured loans | London Interbank Offered Rate (LIBOR)          
Debt          
Variable rate, spread (as a percent)     1.85%    
Variable rate secured loans | Revolving Credit Facility          
Debt          
Weighted average interest rate (as a percent) 1.62%   1.62%    
Revolving Credit Facility          
Debt          
Carrying Value $ 58,000,000   $ 58,000,000   43,500,000
Description of variable rate basis     LIBOR    
Revolving Credit Facility | Minimum | London Interbank Offered Rate (LIBOR)          
Debt          
Variable rate, spread (as a percent)     0.875%    
Revolving Credit Facility | Maximum | London Interbank Offered Rate (LIBOR)          
Debt          
Variable rate, spread (as a percent)     1.60%    
Term Loan Facilities          
Debt          
Carrying Value 516,508,000   $ 516,508,000   515,902,000
Increase to maximum borrowing capacity 150,000,000   150,000,000    
Aggregate additional borrowing capacity available $ 430,000,000   $ 430,000,000    
Description of variable rate basis     LIBOR    
Weighted average interest rate (as a percent) 2.10%   2.10%    
Term Loan Facilities | Minimum | London Interbank Offered Rate (LIBOR)          
Debt          
Variable rate, spread (as a percent)     0.90%    
Term Loan Facilities | Maximum | London Interbank Offered Rate (LIBOR)          
Debt          
Variable rate, spread (as a percent)     2.60%    
3.60% Senior Notes | Unsecured senior notes          
Debt          
Carrying Value $ 346,920,000   $ 346,920,000   346,714,000
Stated interest rate (as a percent) 3.60%   3.60%    
Debt instrument, face amount $ 350,000   $ 350,000    
Unamortized discount included in carrying value $ 2,100,000   $ 2,100,000   2,200,000
Interest rate on debt (as a percent) 3.70%   3.70%    
5.250% Senior Notes | Unsecured senior notes          
Debt          
Carrying Value $ 245,951,000   $ 245,951,000   245,731,000
Stated interest rate (as a percent) 5.25%   5.25%    
Debt instrument, face amount $ 250,000   $ 250,000    
Unamortized discount included in carrying value $ 3,600,000   $ 3,600,000   3,800,000
Interest rate on debt (as a percent) 5.49%   5.49%    
3.70% Senior Notes | Unsecured senior notes          
Debt          
Carrying Value $ 297,609,000   $ 297,609,000   297,378,000
Stated interest rate (as a percent) 3.70%   3.70%    
Debt instrument, face amount $ 300,000   $ 300,000    
Unamortized discount included in carrying value $ 1,900,000   $ 1,900,000   2,100,000
Interest rate on debt (as a percent) 3.85%   3.85%    
5.000% Senior Notes | Unsecured senior notes          
Debt          
Carrying Value $ 296,192,000   $ 296,192,000   296,019,000
Stated interest rate (as a percent) 5.00%   5.00%    
Debt instrument, face amount $ 300,000   $ 300,000    
Unamortized discount included in carrying value $ 3,200,000   $ 3,200,000   3,300,000
Interest rate on debt (as a percent) 5.15%   5.15%    
Unsecured notes payable          
Debt          
Carrying Value $ 1,455,000   $ 1,455,000   1,508,000
Stated interest rate (as a percent) 0.00%   0.00%    
Unamortized discount included in carrying value $ 506,000   $ 506,000   $ 554,000