Summary of Significant Accounting Policies Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans, net	$ 267,093	$ 276,650
Unsecured notes, net	2,283,513	2,280,286
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	267,298	286,684
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	$ 2,359,998	$ 2,340,762