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Secured and Unsecured Debt (Details 3) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Summary of unsecured debt      
Borrowings outstanding at end of period $ 840,500    
Unsecured Debt, Total 2,073,444 1,979,198  
Unamortized discount 15,600   16,900
Borrowings outstanding under an unsecured credit facility due October 2015 [Member]
      
Summary of unsecured debt      
Borrowings outstanding at end of period   76,000  
4.63% Medium-Term Notes due January 2022 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 396,849 396,759  
Fixed interest rate 4.63% 4.63%  
Unamortized discount 3,151 3,241  
1.67% Term Notes due December 2016 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 35,000 35,000  
Fixed interest rate 1.64%   1.65%
2.68% Term Notes due December 2016 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 65,000 65,000  
Fixed interest rate 2.68% 2.68%  
6.05% Medium-Term Notes due June 2013 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 122,500 122,500  
Fixed interest rate 6.05%   6.05%
5.13% Medium-Term Notes due January 2014 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 184,000 184,000  
Fixed interest rate 5.13%   5.13%
5.50% Medium-Term Notes due April 2014 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 128,429 128,411  
Fixed interest rate 5.50%   5.50%
Unamortized discount 71   89
5.25% Medium-Term Notes due January 2015 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 324,945 324,913  
Fixed interest rate 5.25%   5.25%
Unamortized discount 230   262
5.25% Medium-Term Notes due January 2016 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 83,260 83,260  
Fixed interest rate 5.25%   5.25%
2.90% Term Notes due January 2016 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 250,000 250,000  
Fixed interest rate 2.90%   2.90%
8.50% Debentures due September 2024 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 15,644 15,644  
Fixed interest rate 8.50%   8.50%
4.25% Medium-Term Notes due June 2018 [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes 297,785 297,678  
Fixed interest rate 4.25%   4.25%
Unamortized discount 2,215   2,322
Other [Member]
      
Summary of unsecured debt      
Senior Unsecured Notes $ 32 $ 33