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Significant Accounting Policies - Property Plant and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Property, Plant and Equipment [Line Items]      
Depreciation and amortization $ 32.6 $ 24.2 $ 16.8
Textbook Library      
Property, Plant and Equipment [Line Items]      
Useful life 4 years    
Content      
Property, Plant and Equipment [Line Items]      
Useful life 5 years    
Leasehold improvements      
Property, Plant and Equipment [Line Items]      
Useful life 5 years    
Internal-use software and website development      
Property, Plant and Equipment [Line Items]      
Useful life 3 years    
Furniture and fixtures      
Property, Plant and Equipment [Line Items]      
Useful life 5 years    
Computers and equipment      
Property, Plant and Equipment [Line Items]      
Useful life 3 years