Indebtedness - Summary of Indebtedness at Estimated Fair Value (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Amount	[1]	$ 1,304,849	$ 1,353,679
Fair Value		1,341,534	1,384,111
Mortgage Loans Payable
Debt Instrument [Line Items]
Carrying Amount	[1]	451,862	498,861
Fair Value		467,303	513,540
Senior Unsecured Notes
Debt Instrument [Line Items]
Carrying Amount	[1]	248,487	205,318
Fair Value		269,731	222,469
Unsecured Term Loans
Debt Instrument [Line Items]
Carrying Amount	[1]	460,000	460,000
Fair Value		460,000	458,602
Unsecured Credit Facility
Debt Instrument [Line Items]
Carrying Amount	[1]	144,500	189,500
Fair Value		$ 144,500	$ 189,500

[1] The carrying amounts include unamortized premiums and discounts and exclude unamortized debt issuance costs.