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Summary of Significant Accounting Policies - Impairments (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]      
Impairment of goodwill $ 0 $ 0 $ 0
Impairment of unconsolidated entities 0 0 0
Impairment of leasehold improvements and property and equipment $ 0 $ 0 $ 0
Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Length of time for properties more likely than not of being disposed 12 months    
Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Length of time for properties more likely than not of being disposed 24 months