Secured and Unsecured Debt (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Summary of unsecured debt
Borrowings outstanding at end of period	$ 2,200
Unsecured Debt, Total	2,228,820	2,081,626
Unamortized discount	8,000	11,800
Borrowings outstanding under an unsecured credit facility due October 2015 [Member]
Summary of unsecured debt
Borrowings outstanding at end of period	160,000	0
4.63% Medium-Term Notes due January 2022 [Member]
Summary of unsecured debt
Senior Unsecured Notes	397,387	397,118
Fixed interest rate	4.63%	4.63%
Unamortized discount	2,792	2,882
1.67% Term Notes due December 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	35,000	35,000
Fixed interest rate	1.30%	1.65%
2.68% Term Notes due December 2016 [Member]
Summary of unsecured debt
Senior Unsecured Notes	100,000	65,000
Fixed interest rate	2.50%	2.68%
6.05% Medium-Term Notes due June 2013 [Member]
Summary of unsecured debt
Fixed interest rate	6.05%	6.05%
5.13% Medium-Term Notes due January 2014 [Member]
Summary of unsecured debt
Senior Unsecured Notes	0	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	0	128,480
Fixed interest rate	5.50%	5.50%
Unamortized discount	3	20
5.25% Medium-Term Notes due January 2015 [Member]
Summary of unsecured debt
Senior Unsecured Notes	325,138	325,041
Fixed interest rate	5.25%	5.25%
Unamortized discount	102	134
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	215,000	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	298,428	298,107
Fixed interest rate	4.25%	4.25%
Unamortized discount	1,786	1,893
Other [Member]
Summary of unsecured debt
Senior Unsecured Notes	$ 28	$ 30

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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