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Summary of Significant Accounting Policies - Leasehold Improvements and Property and Equipment (Details) - Computer Hardware, Software, Furniture and Fixtures [Member]
12 Months Ended
Dec. 31, 2020
Minimum [Member]  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment estimated useful life 3 years
Maximum [Member]  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment estimated useful life 7 years