Secured and Unsecured Debt (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Summary of unsecured debt
Borrowings outstanding at end of period	$ 1,500	$ 1,900
Unsecured Debt, Total	2,145,996	2,221,576
Unamortized discount	7,000	6,700
Borrowings outstanding under an unsecured credit facility due October 2015 [Member]
Summary of unsecured debt
Borrowings outstanding at end of period	389,000	152,500
4.63% Medium-Term Notes due January 2022 [Member]
Summary of unsecured debt
Senior Unsecured Notes	397,567	397,477
Unamortized discount	2,433	2,523
1.67% Term Notes due December 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	35,000	35,000
2.68% Term Notes due December 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	100,000	100,000
5.25% Medium-Term Notes due January 2015 [Member]
Summary of unsecured debt
Senior Unsecured Notes	0	325,169
Unamortized discount	0	6
5.25% Medium-Term Notes due January 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	83,260	83,260
2.90% Term Notes due January 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	215,000	215,000
8.50% Debentures due September 2024 [Member]
Summary of unsecured debt
Senior Unsecured Notes	15,644	15,644
4.25% Medium-Term Notes due June 2018 [Member]
Summary of unsecured debt
Senior Unsecured Notes	298,642	298,535
Unamortized discount	1,358	1,465
Other [Member]
Summary of unsecured debt
Senior Unsecured Notes	$ 26	$ 27