Redeemable Convertible Preferred Stock (Details) - $ / shares		1 Months Ended		9 Months Ended
	Jul. 30, 2018	Dec. 31, 2015	Oct. 31, 2012	Sep. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, authorized (in shares)	500,000,000			500,000,000	93,855,000
Preferred stock, authorized (in shares)	10,000,000
Preferred stock, issued (in shares)				0
Preferred stock, outstanding (in shares)				0
Series A Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Stock sold (in shares)		15,847,500	15,847,500
Redeemable convertible preferred stock, par value (in usd per share)		$ 0.01		$ 0.01	$ 0.01
Redeemable convertible preferred stock, shares authorized (in shares)				0	15,848,000
Series B Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Stock sold (in shares)		39,538,354
Redeemable convertible preferred stock, par value (in usd per share)		$ 0.01		$ 0.01	$ 0.01
Redeemable convertible preferred stock, shares authorized (in shares)		42,000,000		0	42,000,000
Common Stock
Class of Stock [Line Items]
Issuance of common stock upon conversion of redeemable convertible preferred stock (in shares)				55,386,000
Common Stock | IPO
Class of Stock [Line Items]
Stock sold (in shares)	12,535,000
Issuance of common stock upon conversion of redeemable convertible preferred stock (in shares)	55,385,854