Convertible Preferred Stock and Stockholders' Equity - Summary of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 02, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class Of Stock [Line Items]
Balance, January 1, 2018				$ (39,454,051)
Balance, January 1, 2018				137,762,160
Conversion of convertible preferred stock into common stock upon initial public offering				137,762,160
Convertible preferred shares converted to common stock	24,168,656
Sale of common stock in initial public offering				150,843,215
Vesting of common stock from option exercise				56,986
Issuance of common stock from option exercise				$ 45,206
Issuance of common stock from option exercise, Shares				73,296		0
Stock-based compensation				$ 1,667,790
Comprehensive gain		$ 27,768		52,430
Net loss		(14,795,763)	$ (6,527,568)	(38,213,561)	$ (18,085,115)
Balance, September 30, 2018		212,760,175		$ 212,760,175		$ (39,454,051)
Balance, June 30, 2018						$ 137,762,160
Convertible Preferred Stock
Class Of Stock [Line Items]
Balance, Shares, January 1, 2018				127,199,705
Balance, January 1, 2018				$ 137,762,160
Conversion of convertible preferred stock into common stock upon initial public offering, Shares				(127,199,705)
Conversion of convertible preferred stock into common stock upon initial public offering				$ (137,762,160)
Balance, Shares, June 30, 2018						127,199,705
Balance, June 30, 2018						$ 137,762,160
Common Stock
Class Of Stock [Line Items]
Balance, January 1, 2018				$ 264
Balance, Shares, January 1, 2018				2,637,011
Conversion of convertible preferred stock into common stock upon initial public offering				$ 2,417
Convertible preferred shares converted to common stock				24,168,656
Sale of common stock in initial public offering				$ 1,035
Shares issued and sold				10,350,000
Vesting of common stock from option exercise				$ 11
Vesting of common stock from option exercise, Shares				97,250
Issuance of common stock from option exercise				$ 6
Issuance of common stock from option exercise, Shares				57,146
Balance, September 30, 2018		$ 3,733		$ 3,733		$ 264
Balance, Shares, June 30, 2018		37,310,063		37,310,063		2,637,011
Additional Paid-in Capital
Class Of Stock [Line Items]
Balance, January 1, 2018				$ 799,859
Conversion of convertible preferred stock into common stock upon initial public offering				137,759,743
Sale of common stock in initial public offering				150,842,180
Vesting of common stock from option exercise				56,975
Issuance of common stock from option exercise				45,200
Stock-based compensation				1,667,790
Balance, September 30, 2018		$ 291,171,747		291,171,747		$ 799,859
Accumulated Other Comprehensive Loss
Class Of Stock [Line Items]
Balance, January 1, 2018				(73,308)
Comprehensive gain				52,430
Balance, September 30, 2018		(20,878)		(20,878)		(73,308)
Accumulated Deficit
Class Of Stock [Line Items]
Balance, January 1, 2018				(40,180,866)
Net loss				(38,213,561)
Balance, September 30, 2018		$ (78,394,427)		$ (78,394,427)		$ (40,180,866)