Fair Value Measurements - Financial Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Mortgages payable | Carrying Amount
Financial liabilities:
Financial liabilities	$ 1,361,840	$ 1,844,652
Mortgages payable | Fair Value
Financial liabilities:
Financial liabilities	1,404,849	1,870,078
Unsecured term loans | Carrying Amount
Financial liabilities:
Financial liabilities	300,000	300,000
Unsecured term loans | Fair Value
Financial liabilities:
Financial liabilities	$ 301,076	$ 300,727