Debt (Details) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Apr. 20, 2015
Dec. 31, 2014
Debt        
Carrying Value $ 1,999,622,000     $ 1,920,057,000
Interest costs capitalized 2,100,000 1,600,000    
Wholly owned subsidiary        
Debt        
Debt Instrument, Debt Default, Amount 150,000,000.0      
Mortgage and Other Secured Loans:        
Debt        
Carrying Value 422,361,000     424,016,000
Fixed rate mortgage loans        
Debt        
Carrying Value 385,693,000     387,139,000
Stated interest rates, low end of range (as a percent) 3.96%      
Stated interest rates, high end of range (as a percent) 10.65%      
Unamortized premium included in carrying value 36,000     42,000
Weighted average interest rate (as a percent) 8.11%      
Debt, Weighted Average Interest Rate, Excluding Incremental Rate On Default Rate 6.16%      
Fixed rate mortgage loans | Maximum        
Debt        
Interest rate excluding incremental rate on default rate (percent) 7.87%      
Variable rate secured loans        
Debt        
Carrying Value 36,668,000     36,877,000
Description of variable rate basis LIBOR      
Stated interest rate (as a percent) 2.42%      
Variable rate secured loans | London Interbank Offered Rate (LIBOR) [Member]        
Debt        
Variable rate, spread (as a percent) 2.25%      
Variable rate secured loans | Revolving Credit Facility        
Debt        
Weighted average interest rate (as a percent) 1.45%      
Revolving Credit Facility        
Debt        
Carrying Value 164,000,000     83,000,000
Description of variable rate basis LIBOR      
Revolving Credit Facility | Minimum | London Interbank Offered Rate (LIBOR) [Member]        
Debt        
Variable rate, spread (as a percent) 0.975%      
Revolving Credit Facility | Maximum | London Interbank Offered Rate (LIBOR) [Member]        
Debt        
Variable rate, spread (as a percent) 1.75%      
Revolving Credit Facility | Revolving Credit Facility        
Debt        
Maximum Availability 800,000,000      
Term Loan Facilities        
Debt        
Carrying Value 520,000,000     520,000,000
Description of variable rate basis LIBOR      
Weighted average interest rate (as a percent) 1.82%      
Additional borrowing capacity available provided there is no default under the agreement 330,000,000      
Term Loan Facilities | Minimum | London Interbank Offered Rate (LIBOR) [Member]        
Debt        
Variable rate, spread (as a percent) 1.10%      
Term Loan Facilities | Maximum | London Interbank Offered Rate (LIBOR) [Member]        
Debt        
Variable rate, spread (as a percent) 2.60%      
3.60% Senior Notes | Unsecured senior notes        
Debt        
Carrying Value 347,560,000     347,496,000
Stated interest rate (as a percent) 3.60%      
Interest rate on debt (as a percent) 3.70%      
Debt instrument, face amount 350,000,000      
Unamortized discount included in carrying value 2,400,000     2,500,000
5.250% Senior Notes | Unsecured senior notes        
Debt        
Carrying Value 245,888,000     245,797,000
Stated interest rate (as a percent) 5.25%      
Interest rate on debt (as a percent) 5.49%      
Debt instrument, face amount 250,000,000      
Unamortized discount included in carrying value 4,100,000     4,200,000
3.70% Senior Notes | Unsecured senior notes        
Debt        
Carrying Value 297,655,000     297,569,000
Stated interest rate (as a percent) 3.70%      
Interest rate on debt (as a percent) 3.85%      
Debt instrument, face amount 300,000,000      
Unamortized discount included in carrying value 2,300,000     2,400,000
Unsecured notes payable        
Debt        
Carrying Value 1,583,000     1,607,000
Stated interest rate (as a percent) 0.00%      
Unamortized discount included in carrying value 628,000     654,000
4.25% Exchangeable Senior Notes        
Debt        
Carrying Value 575,000     572,000
Stated interest rate (as a percent) 4.25%      
Interest rate on debt (as a percent) 6.05%      
Unamortized discount included in carrying value       3,000
Exchange rate per $1000 principal amount (in common shares) 20.8513      
Convertible debt principal amount, basis for exchange 1,000      
Exchange price per common share based on exchange rate (in dollars per share) $ 47.96      
Principal amount of debt       $ 575,000
4.25% Exchangeable Senior Notes | Subsequent Event        
Debt        
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed     100.00%  
Debt in default of payment terms | Wholly owned subsidiary        
Debt        
Stated interest rate (as a percent) 10.65%      
Debt in default of payment terms | Wholly owned subsidiary | Northern Virginia        
Debt        
Number of real estate properties 2