Provision for Impairment of Investment Properties - Impairment Charges (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($) ft²	Dec. 31, 2019 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items]
Provision for impairment of investment properties	$ 2,279	$ 11,177	$ 2,625	$ 11,177	$ 12,298
Estimated fair value of impaired properties as of impairment date			$ 14,144	$ 5,300
King Philip's Crossing
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²	105,900		105,900
Provision for impairment of investment properties			$ 346
Streets of Yorktown
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²	85,200	85,200	85,200	85,200
Provision for impairment of investment properties			$ 2,279	$ 11,177