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Convertible Preferred Stock and Stockholders' Deficit - Summary of Common Stock Issued and Reserved for Issuance (Parenthetical) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Series A-2 Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock converted to common stock, ratio	3.539%	3.539%
Series B Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock converted to common stock, ratio	4.103%	4.103%
Series C Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock converted to common stock, ratio	5.071%	5.071%