Convertible Preferred Stock - Narrative (Details) $ / shares in Units, $ in Millions		1 Months Ended						12 Months Ended
	Dec. 07, 2017 $ / shares shares	Nov. 30, 2017 USD ($) $ / shares shares	Aug. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Jan. 31, 2016 USD ($) shares	Jul. 31, 2015 USD ($) shares	May 31, 2015 USD ($) director shares	Dec. 31, 2019 $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2018 $ / shares shares
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock, par value (usd per share) | $ / shares								$ 0.01		$ 0.01
Common stock, par value (usd per share) | $ / shares								$ 0.01		$ 0.01
Dividends declared and accrued | $									$ 0.0
Conversion ratio								100.00%
Underwritten IPO stock price (in usd per share) | $ / shares	$ 5.00
Underwritten IPO gross proceeds | $									$ 50.0
Convertible preferred stock, shares issued (in shares)								0		0
Convertible preferred stock, shares outstanding (in shares)								0		0
First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock, par value (usd per share) | $ / shares				$ 0.01
Common stock, par value (usd per share) | $ / shares				$ 0.01
Common Stock
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock converted into shares of common stock (in shares)	60,365,020								60,365,020
Common Stock | First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				81,787,360
Preferred Stock | First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				63,288,470
Series A-1 Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Dividends entitled to be received by preferred stockholders (in use per share) | $ / shares	$ 0.08
Series A-1 Convertible Preferred Stock | Initial Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)							45,223,970
Number of stock issued (in shares)							6,295,810
Net proceeds | $							$ 24.8
Series A-1 Convertible Preferred Stock | First Additional Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)							456,250
Number of stock issued (in shares)							3,481,250
Net proceeds | $							$ 13.9
Series A-1 Convertible Preferred Stock | Second Additional Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)						2,420,830
Net proceeds | $						$ 9.6
Series A-1 Convertible Preferred Stock | Third Additional Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)					500,000
Net proceeds | $					$ 2.0
Series A-1 Convertible Preferred Stock | First Tranche Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)					11,250,000
Net proceeds | $					$ 45.0
Series A-1 Convertible Preferred Stock | Second Tranche Closing and Series A-2 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)				22,166,550
Net proceeds | $				$ 88.7
Series A-1 Convertible Preferred Stock | First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				46,114,433
Series A-2 Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Dividends entitled to be received by preferred stockholders (in use per share) | $ / shares	0.16
Series A-2 Convertible Preferred Stock | Initial Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)							4,361,530
Series A-2 Convertible Preferred Stock | Second Tranche Closing and Series A-2 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)				4,361,530
Net proceeds | $				$ 34.9
Series A-2 Convertible Preferred Stock | First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				4,361,533
Series B-1 Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Dividends entitled to be received by preferred stockholders (in use per share) | $ / shares	0.32
Series B-1 Convertible Preferred Stock | First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				8,125,000
Number of stock issued (in shares)				7,646,240
Closing stock price (usd per share) | $ / shares				$ 16.00
Net proceeds | $				$ 122.2
Series B-1 Convertible Preferred Stock | Second Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)			478,120
Closing stock price (usd per share) | $ / shares			$ 16.00
Net proceeds | $			$ 7.6
Series B-2 Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)		1,764,710
Closing stock price (usd per share) | $ / shares		$ 17.00
Net proceeds | $		$ 29.9
Dividends entitled to be received by preferred stockholders (in use per share) | $ / shares	$ 0.34
Series B-2 Convertible Preferred Stock | First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				4,687,500
Series A
Subsidiary, Sale of Stock [Line Items]
Minimum shares held by investors (in shares)							100,000
Number of members under board election | director							1
Total number of members | director							5
Series B
Subsidiary, Sale of Stock [Line Items]
Number of members under board election | director							1
Series A and Series B
Subsidiary, Sale of Stock [Line Items]
Additional number of members under board election | director							2