Secured and Unsecured Debt (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Summary of unsecured debt
Borrowings outstanding at end of period	$ 839,700
Senior Unsecured Notes	250,000
Unsecured Debt, Total	2,009,689	1,979,198
Unamortized discount	14,400		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	396,939	396,759
Fixed interest rate	4.63%	4.63%
Unamortized discount	3,061	3,241
1.67% Term Notes due December 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	35,000	35,000
Fixed interest rate	1.45%		1.65%
2.68% Term Notes due December 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	65,000	65,000
Fixed interest rate	2.50%	2.68%
6.05% Medium-Term Notes due June 2013 [Member]
Summary of unsecured debt
Senior Unsecured Notes	0	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	128,446	128,411
Fixed interest rate	5.50%		5.50%
Unamortized discount	54		89
5.25% Medium-Term Notes due January 2015 [Member]
Summary of unsecured debt
Senior Unsecured Notes	324,977	324,913
Fixed interest rate	5.25%		5.25%
Unamortized discount	198		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	2.73%		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	297,892	297,678
Fixed interest rate	4.25%		4.25%
Unamortized discount	2,108		2,322
Other [Member]
Summary of unsecured debt
Senior Unsecured Notes	$ 31	$ 33