Fair Value of Financial Instruments (Details) (Fair Value, Inputs, Level 3 [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Portion at Other than Fair Value Measurement [Member] | Unsecured Debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Long-term Debt, Fair Value	$ 2,509,094	$ 2,708,213
Portion at Other than Fair Value Measurement [Member] | Mortgage Loans on Real Estate [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Long-term Debt, Fair Value	487,301	545,306
Estimate of Fair Value Measurement [Member] | Unsecured Debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Long-term Debt, Fair Value	2,704,069	2,764,831
Estimate of Fair Value Measurement [Member] | Mortgage Loans on Real Estate [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Long-term Debt, Fair Value	$ 501,231	$ 573,944