Summary of Significant Accounting Policies (Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans, net	$ 276,650	$ 307,908
Unsecured notes, net	2,280,286	2,580,108
Secured Debt [Member] | Fair Value, Inputs, Level 3 [Member] | Long-term Debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value	286,684	306,334
Unsecured Debt [Member] | Fair Value, Inputs, Level 3 [Member] | Long-term Debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value	$ 2,340,762	$ 2,616,395