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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, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
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 $ 68,997,000 $ 68,539,000 $ 66,813,000 $ 64,716,000 $ 66,578,000 $ 68,705,000 $ 66,694,000 $ 66,893,000 $ 269,065,000 $ 268,870,000 $ 272,588,000
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                 $ 200,000 $ 500,000 $ 600,000