Provision for Impairment of Investment Properties - Impairment Charges (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) ft²	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) ft²	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items]
Provision for impairment of investment properties	$ 0	$ 0	$ 346	$ 0	$ 12,298
Estimated fair value of impaired properties as of impairment date			$ 11,644
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