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Summary of Significant Accounting Policies (Accounts Receivable/Deferred Rent Receivable) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]      
Allowance for doubtful accounts receivable $ 6.4 $ 6.6  
Provision for doubtful accounts $ 1.4 $ 0.6 $ 0.6