Fair Value Measurements - Financial Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Mortgages Payable | Fair Value
Financial liabilities:
Financial liabilities	$ 1,614,367	$ 1,606,470
Mortgages Payable | Carrying Amount
Financial liabilities:
Financial liabilities	1,601,635	1,603,869
Unsecured term loans | Fair Value
Financial liabilities:
Financial liabilities	400,354	399,678
Unsecured term loans | Carrying Amount
Financial liabilities:
Financial liabilities	$ 400,000	$ 400,000