Investments in Real Estate - Investments in Existing Properties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments in real estate properties
Capitalized costs on existing portfolio	$ 8,708	$ 23,536	$ 25,350
Investments in existing properties
Investments in real estate properties
Capitalized costs on existing portfolio	7,000	17,900
Re-leasing costs	1,800	2,100
Recurring capital expenditures	198	801
Nonrecurring building improvements	$ 5,000	$ 15,000