Common Shares (Details) $ / shares in Units, $ in Thousands			9 Months Ended	12 Months Ended
	Jun. 22, 2018 USD ($) $ / shares shares	May 29, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) director Vote $ / shares shares	Dec. 31, 2017 USD ($) director $ / shares shares	May 31, 2018 $ / shares shares	Feb. 28, 2018 shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)				43,918,239	200,000,000	44,746,463
Common stock, par value (in dollars per share) | $ / shares				$ 0.000273235	$ 0.000273235
Common shares or Preferred Shares not designated				11,956,456
Proceeds form IPO | $			$ 155,536
Authorized capital					54,647,000
Preferred shares designated			0
Preferred stock, shares issued (in shares)			0
Common stock, cash dividends declared | $			$ 0	$ 0
Common stock, cash dividends paid | $			0	0
Preferred stock, cash dividends declared | $			0	0
Preferred stock, cash dividends paid | $			$ 0	$ 0
Class A common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)				5,507,938		5,507,938
Common stock, par value (in dollars per share) | $ / shares			$ 0.000273235	$ 0.000273235
Number of shares issued on conversion of convertible preferred shares	5,546,019		5,546,019		5,546,019
Number of votes (per share) | Vote			1
Class B common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)				3,568,353		3,568,353
Common stock, par value (in dollars per share) | $ / shares			$ 0.000273235	$ 0.000273235
Number of shares issued on conversion of convertible preferred shares	1,070,502		1,070,502		1,070,502
Number of votes (per share) | Vote			10
Convertible ratio to Class A common share			1
Convertible ratio to Class B1 common share			1
Class A1 common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)				0
Number of shares issued on conversion of convertible preferred shares			12,995,954		12,995,954
Number of votes (per share) | Vote			0
Convertible ratio to Class A common share			1
Class B1 common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)				0
Number of shares issued on conversion of convertible preferred shares			16,057,618		16,057,618
Number of votes (per share) | Vote			0
Convertible ratio to Class A common share			1
Convertible ratio to Class B common share			1
Common Class A and Class B
Nature of the Business and Basis of Presentation
Number of directors entitled to elect | director			1	1
Convertible Preferred Shares (Series A, B and C)
Nature of the Business and Basis of Presentation
Preferred shares designated			0	22,885,492		35,670,093
Preferred stock, shares issued (in shares)			0	22,885,492
Number of directors entitled to elect | director			2	2
Initial Public Offering | Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)		8,477,777
Share price (in dollars per share) | $ / shares		$ 18.00
Proceeds form IPO | $		$ 152,600
Over-allotment | Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)	1,006,425
Share price (in dollars per share) | $ / shares	$ 18.00
Proceeds form IPO | $	$ 18,116