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Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs (Details) - entity
Mar. 31, 2018
Dec. 31, 2017
Related Party Transactions and Investments in Non-Consolidated Entities    
Number of REITs in which the entity holds non-controlling common stock interest 6 6
Number of REITs in which the entity holds non-controlling preferred stock interest 2