Fair Value Measurements - Financial Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Mortgages payable | Carrying Amount
Financial liabilities:
Financial liabilities	$ 1,127,848	$ 1,844,652
Mortgages payable | Fair Value
Financial liabilities:
Financial liabilities	1,162,890	1,870,078
Revolving Credit Facility [Member] | Carrying Amount
Financial liabilities:
Financial liabilities	200,000	0
Revolving Credit Facility [Member] | Fair Value
Financial liabilities:
Financial liabilities	200,177	0
Unsecured term loans | Carrying Amount
Financial liabilities:
Financial liabilities	300,000	300,000
Unsecured term loans | Fair Value
Financial liabilities:
Financial liabilities	$ 300,607	$ 300,727