Convertible Preferred Shares - Additional Information (Detail)				1 Months Ended		9 Months Ended		12 Months Ended
	Jul. 17, 2018 USD ($) $ / shares shares	Oct. 20, 2017	Jun. 30, 2017 $ / shares shares	Jul. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares
Class Of Stock [Line Items]
Preferred stock, shares authorized | shares						10,000,000			10,000,000
Preferred stock, par value | $ / shares						$ 0.001			$ 0.001
Share conversion ratio			1
Conversion of bridge units in to preferred units							$ 8,500,000
Common Stock [Member]
Class Of Stock [Line Items]
Reverse stock split ratio		0.164544
Reverse stock split		One-for-6.0774
Redeemable Convertible Preferred Shares [Member]
Class Of Stock [Line Items]
Redeemable convertible shares, authorized | shares			43,297,267
Redeemable convertible shares, par value | $ / shares			$ 0.001
Redeemable Convertible Preferred Shares [Member] | Class C Preferred Units [Member]
Class Of Stock [Line Items]
Redeemable convertible shares, par value | $ / shares					$ 1.7749
Redeemable convertible shares issued and sold | shares					24,326,470			5,321,112
Proceeds from issuance of preferred units, net of issuance costs					$ 43,000,000
Payment of offering costs					$ 200,000
Preferred units discount percentage								10.00%
Conversion of bridge units in to preferred units								$ 8,500,000
Conversion price of redeemable preferred share | $ / shares								$ 1.60
Convertible debt fair value disclosures								$ 9,400,000
Liability fair value adjustment								8,500,000
Redeemable issuer option value								900,000
Gain on extinguishment								$ 0
Redeemable Convertible Preferred Shares [Member] | Class C Preferred Units [Member] | Chief Financial Officer [Member]
Class Of Stock [Line Items]
Redeemable convertible shares, par value | $ / shares				$ 1.7749
Redeemable convertible shares issued and sold | shares				61,880
Proceeds from issuance of preferred units, net of issuance costs				$ 100,000
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Payment of offering costs	$ 1,000,000
Underwritten Public Offering [Member] | Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares | shares	3,780,000
Share price | $ / shares	$ 12.50
Underwritten Public Offering [Member] | Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares | shares	2,220
Share price | $ / shares	$ 12,500
Number of shares issuable upon conversion | shares	1,000					1,000
Percentage of shares owned by share holders						9.99%
Preferred stock, liquidation per share | $ / shares						$ 0.001
Voting rights						no