Unsecured Debt and Lines of Credit (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Unsecured debts and line of credit [Abstract]
Unsecured debt	$ 1,410,162	$ 1,410,023
Lines of credit	135,903	219,421
Total unsecured debt and lines of credit	1,546,065	1,629,444
Bonds Private Placement - Fixed Rate [Member]
Unsecured debts and line of credit [Abstract]
Unsecured debt	465,000	465,000
Weighted Average Maturity	5 years
Term Loan - Variable Rate [Member]
Unsecured debts and line of credit [Abstract]
Unsecured debt	350,000	350,000
Weighted Average Maturity	2 years 10 months 24 days
Weighted average interest rate (in hundredths)	2.40%	2.50%
Bonds Public Offering - Fixed Rate [Member]
Unsecured debts and line of credit [Abstract]
Unsecured debt	$ 595,162	$ 595,023
Weighted Average Maturity	8 years 8 months 12 days
Weighted average interest rate (in hundredths)	4.00%	4.00%
Line of Credit [Member]
Unsecured debts and line of credit [Abstract]
Weighted Average Maturity, Line of credit	5 years
Weighted average interest rate (in hundredths)	1.60%	2.20%

Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer) and 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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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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