Related Party Transactions and Investments in Non-Consolidated Entities - Management fees and interest income from loans (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Asset management fees, low end of range (as a percent)									1.00%
Asset management fees, high end of range (as a percent)									5.00%
Notice period for cancellation of applicable contracts									30 days
Revenue	$ 59,810,000	$ 62,247,000	$ 60,808,000	$ 62,983,000	$ 68,997,000	$ 68,539,000	$ 66,813,000	$ 64,716,000
Revenue, Product and Service [Extensible List]									Asset management fees	Asset management fees	Asset management fees
Impairment of Sponsored REIT	$ 0								$ 0
Asset management fees
Schedule of Equity Method Investments [Line Items]
Revenue									$ 100,000	$ 200,000	$ 500,000