Fair Value Measurements - Fair Value of Financial Instruments Presented at Carrying Values (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Value
Debt Instrument [Line Items]
Mortgages payable	$ 292,835	$ 370,804
Line of credit and term loan	300,000	300,000
Estimated Fair Value
Debt Instrument [Line Items]
Mortgages payable	291,144	372,962
Line of credit and term loan	$ 299,789	$ 299,770