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Notes Payable (Narrative) (Details)
$ in Thousands
 1 Months Ended 4 Months Ended 12 Months Ended
May 31, 2017
Feb. 28, 2017
USD ($)
Aug. 31, 2015
USD ($)
May 15, 2017
USD ($)
Dec. 31, 2017
USD ($)
yr
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
May 12, 2017
USD ($)
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net         $ 12,300 $ 15,700    
Unsecured Debt         1,338,628 1,583,236    
Total notes payable         2,204,600 2,480,600    
Loss on early retirement of debt         323 0 $ 0  
Available amount under unsecured credit facility         $ 586,600      
Repayments of Secured Debt   $ 30,700            
Weighted average interest rate on floating rate debt [1]         4.30%      
Weighted average maturity of indebtedness (including unsecured credit facility and unsecured short-term borrowings) (in years) | yr         4.3      
Short-term Debt [Member]                
Maximum borrowing capacity under unsecured credit facility               $ 45,000
Debt Instrument, Maturity Date         May 14, 2018      
Debt Instrument, Basis Spread on Variable Rate 0.95%              
Available amount under unsecured credit facility         $ 45,000      
Other Short-term Borrowings         0      
Letter Of Credit [Member]                
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility         50,000      
Outstanding balance under credit facility         13,400      
Senior Unsecured Notes [Member]                
Unsecured Debt         1,338,600 1,583,200    
Five Point Eight three Percentage Notes Due2017 [Member] | Senior Unsecured Notes [Member]                
Unsecured Debt         $ 0 246,600    
Debt Instrument, Maturity Date         May 15, 2017      
Notes payable, effective interest rate         5.83%      
Repayments of Unsecured Debt       $ 246,800        
Unsecured Credit Facility [Member]                
Value not exceeding the amount available under the unsecured credit facility         $ 300,000      
Amount of current borrowing capacity under unsecured credit facility     $ 500,000   600,000      
Maximum borrowing capacity under unsecured credit facility         $ 900,000      
Terms Of Bid Rate Loans         180      
Unsecured Credit Facility, Expiration Date     Sep. 22, 2015   Aug. 07, 2019      
Option to extend the period of unsecured credit facility         August 2020      
Floating Rate Notes Payable [Member]                
Total notes payable         $ 175,000 $ 205,700    
Weighted average interest rate on floating rate debt         1.90% 1.40%    
[1] Includes all available extension options.