Subsequent events - Additional Information (Detail) - Subsequent Event [Member] - IPO [Member] $ / shares in Units, $ in Thousands	1 Months Ended
	Jul. 31, 2018 USD ($) $ / shares shares
Subsequent Event [Line Items]
Proceeds from IPO | $	$ 163,634
Underwriter Options [Member]
Subsequent Event [Line Items]
Common stock, shares issued	1,350,000
Class 2 Common Stock [Member]
Subsequent Event [Line Items]
Common stock, shares issued	10,350,000
Share Price | $ / shares	$ 17.00
Conversion of preferred stock to common stock	7,794,042
Preferred stock conversion basis	One-for-one basis.