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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

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