Fair Value Measurements - Financial Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Mortgages payable | Carrying Amount
Financial liabilities:
Financial liabilities	$ 1,910,726	$ 2,035,959
Mortgages payable | Fair Value
Financial liabilities:
Financial liabilities	1,921,116	2,060,899
Revolving credit facility | Carrying Amount
Financial liabilities:
Financial liabilities	35,729	115,751
Revolving credit facility | Fair Value
Financial liabilities:
Financial liabilities	35,742	115,768
Unsecured term loan | Carrying Amount
Financial liabilities:
Financial liabilities	100,000	50,000
Unsecured term loan | Fair Value
Financial liabilities:
Financial liabilities	$ 100,096	$ 50,029