STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFICIT (Unaudited) - USD ($) $ in Thousands	Total	Series A-1 Preferred Stock	Series A-2 Preferred Stock	Series A-3 Preferred Stock	Series B Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning Balances, Shares at Dec. 31, 2017						10
Stock-based compensation expense	$ 1,268						$ 1,268
Issuance of preferred stock		$ 142,253	$ 140,000
Issuance of preferred stock, Shares		14,225,324	14,000,000
Net loss	(175,216)								$ (175,216)
Redeemable convertible preferred stock, Ending Balance, Shares at Sep. 30, 2018		14,225,324	14,000,000
Redeemable convertible preferred stock, Ending Balance at Sep. 30, 2018		$ 142,253	$ 140,000
Ending Balances, Shares at Sep. 30, 2018						10
Ending Balances at Sep. 30, 2018	(173,948)						1,268		(175,216)
Redeemable convertible preferred stock, Beginning Balance, Shares at Jun. 30, 2018		14,225,324	14,000,000
Redeemable convertible preferred stock, Beginning Balance at Jun. 30, 2018		$ 142,253	$ 140,000
Beginning Balances, Shares at Jun. 30, 2018						10
Beginning Balances at Jun. 30, 2018	(158,846)						688		(159,534)
Stock-based compensation expense	580						580
Net loss	(15,682)								(15,682)
Redeemable convertible preferred stock, Ending Balance, Shares at Sep. 30, 2018		14,225,324	14,000,000
Redeemable convertible preferred stock, Ending Balance at Sep. 30, 2018		$ 142,253	$ 140,000
Ending Balances, Shares at Sep. 30, 2018						10
Ending Balances at Sep. 30, 2018	(173,948)						1,268		(175,216)
Redeemable convertible preferred stock, Beginning Balance, Shares at Dec. 31, 2018		14,225,324	17,000,000	0	0
Redeemable convertible preferred stock, Beginning Balance at Dec. 31, 2018	312,253	$ 142,253	$ 170,000
Beginning Balances, Shares at Dec. 31, 2018						10
Beginning Balances at Dec. 31, 2018	(188,391)						1,879		(190,270)
Stock-based compensation expense	2,218						2,218
Issuance of common stock for stock options exercised	$ 1,402					$ 1	1,401
Issuance of common stock for stock options exercised shares	493,525					493,525
Issuance of common stock upon vesting of RSAs						$ 378,789
Issuance of preferred stock				$ 80,000	$ 75,000
Issuance of preferred stock, Shares				4,705,882	4,687,500
Net loss	$ (74,859)								(74,859)
Other comprehensive loss	(30)							$ (30)
Redeemable convertible preferred stock, Ending Balance, Shares at Sep. 30, 2019		14,225,324	17,000,000	4,705,882	4,687,500
Redeemable convertible preferred stock, Ending Balance at Sep. 30, 2019	467,253	$ 142,253	$ 170,000	$ 80,000	$ 75,000
Ending Balances, Shares at Sep. 30, 2019						872,324
Ending Balances at Sep. 30, 2019	(259,660)					$ 1	5,498	(30)	(265,129)
Redeemable convertible preferred stock, Beginning Balance, Shares at Jun. 30, 2019		14,225,324	17,000,000		4,687,500
Redeemable convertible preferred stock, Beginning Balance at Jun. 30, 2019		$ 142,253	$ 170,000		$ 75,000
Beginning Balances, Shares at Jun. 30, 2019						724,795
Beginning Balances at Jun. 30, 2019	(212,528)					$ 1	4,190		(216,719)
Stock-based compensation expense	931						931
Issuance of common stock for stock options exercised	377						377
Issuance of common stock for stock options exercised shares						132,529
Issuance of common stock upon vesting of RSAs						$ 15,000
Issuance of preferred stock				$ 80,000
Issuance of preferred stock, Shares				4,705,882
Net loss	(48,410)								(48,410)
Other comprehensive loss	(30)							(30)
Redeemable convertible preferred stock, Ending Balance, Shares at Sep. 30, 2019		14,225,324	17,000,000	4,705,882	4,687,500
Redeemable convertible preferred stock, Ending Balance at Sep. 30, 2019	467,253	$ 142,253	$ 170,000	$ 80,000	$ 75,000
Ending Balances, Shares at Sep. 30, 2019						872,324
Ending Balances at Sep. 30, 2019	$ (259,660)					$ 1	$ 5,498	$ (30)	$ (265,129)