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Summary of Significant Accounting Policies - Share-Based Compensation (Details) - $ / shares
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Class of Stock [Line Items]    
Number of days prior to the grant that common stock and restricted stock units fair value is determined based on 3 days  
Class A    
Class of Stock [Line Items]    
Common stock, par value (in dollars per share) $ 0.00001 $ 0.00001