Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($)					1 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 06, 2018	May 21, 2018	Jan. 04, 2018	Dec. 03, 2015	Jan. 31, 2018	Nov. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Stockholders Equity Deficit [Line Items]
Common stock, par value									$ 0.0001	$ 0.0001
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs							$ 291,344,000
Common stock, voting right									one
Shares of common stock, repurchase									5,053,169	7,482,032
Series A Convertible Preferred Stock | Stock Restriction Agreements
Stockholders Equity Deficit [Line Items]
Shares subject to forfeiture						1,305,427
Founder
Stockholders Equity Deficit [Line Items]
Common stock issued shares	1,795,023	251,547
Percentage of fully diluted share capital			15.00%
Increased in equity capital amount			$ 300,000,000
Common stock issued price per share	$ 9.63	$ 2.61
Additional shares of restricted stock subject to vesting restrictions	1,795,023	251,547
Common Stock
Stockholders Equity Deficit [Line Items]
Conversion of convertible preferred stock into common stock, shares							30,493,460		30,493,460
Common stock issued shares							19,837,500
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs							$ 2,000
Unrestricted shares of common stock were subject to service vesting conditions								4,580,444
Common Stock | Stock Restriction Agreements
Stockholders Equity Deficit [Line Items]
Unrestricted shares of common stock were subject to service vesting conditions					4,580,444
Founder Shares
Stockholders Equity Deficit [Line Items]
Common stock issued shares				9,160,888
Common stock, par value				$ 0.0001
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs				$ 4,100
Common Stock vesting percentage			50.00%
Common stock vesting period			5 years