Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended		36 Months Ended
	Feb. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		5,000,000			5,000,000
Preferred stock, shares issued (in shares)		0			0
Preferred stock, shares outstanding (in shares)		0			0
Series A Convertible Preferred Stock
Class of Stock [Line Items]
Issuances of convertible preferred stock (in shares)				47,000,000
Convertible preferred stock, par or stated value (in dollars per share)			$ 1.00
Proceeds from the issuance of convertible preferred stock, net of issuance costs				$ 44.6
Issuance costs				$ 0.1
Series B Convertible Preferred Stock
Class of Stock [Line Items]
Issuances of convertible preferred stock (in shares)			24,778,761
Convertible preferred stock, par or stated value (in dollars per share)			$ 2.26
Proceeds from the issuance of convertible preferred stock, net of issuance costs			$ 55.8
Issuance costs			0.2
Series B-1 Convertible Preferred Stock
Class of Stock [Line Items]
Issuances of convertible preferred stock (in shares)		10,448,100
Convertible preferred stock, par or stated value (in dollars per share)		$ 3.46
Proceeds from the issuance of convertible preferred stock, net of issuance costs		$ 36.1
Issuance costs		$ 0.1
IPO
Class of Stock [Line Items]
Stock converted upon completion of IPO (in shares)	22,283,690
Convertible Promissory Note Receivable | Series A Convertible Preferred Stock
Class of Stock [Line Items]
Outstanding convertible promissory notes			$ 2.3