Stock-based Compensation - Schedule of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Original Stock Option Plan [Member]
Shares
Shares, Beginning Balance	364,571
Shares, Granted	301,442
Shares, Exercised	(2,716)
Shares, Forfeited	(1,051)
Shares, Cancelled	(886)	0
Shares, Ending Balance	661,360	364,571
Weighted-average exercise price
Weighted-average exercise price, Beginning Balance	$ 2.41
Weighted-average exercise price, Granted	5.66
Weighted-average exercise price, Exercised	3.47
Weighted-average exercise price, Forfeited	3.01
Weighted-average exercise price, Cancelled	3.26
Weighted-average exercise price, Ending Balance	$ 3.89	$ 2.41
Aggregate intrinsic value
Aggregate intrinsic value, Beginning Balance	$ 1,185
Aggregate intrinsic value, Ending Balance	$ 1,174	$ 1,185
New Stock Option Plan [Member]
Shares
Shares, Beginning Balance	0
Shares, Granted	5,479,196
Shares, Forfeited	(14,282)
Shares, Ending Balance	5,464,914	0
Weighted-average exercise price
Weighted-average exercise price, Beginning Balance	$ 0.00
Weighted-average exercise price, Granted	7.76
Weighted-average exercise price, Exercised	7.76
Weighted-average exercise price, Forfeited	7.76
Weighted-average exercise price, Ending Balance	$ 7.76	$ 0.00
Aggregate intrinsic value
Aggregate intrinsic value, Beginning Balance	$ 0
Aggregate intrinsic value, Ending Balance	$ 0	$ 0
Performance Shares [Member]
Shares
Shares, Beginning Balance	0
Shares, Granted	600,000
Shares, Ending Balance	600,000	0
Weighted-average exercise price
Weighted-average exercise price, Beginning Balance	$ 0.00
Weighted-average exercise price, Granted	7.76
Weighted-average exercise price, Exercised	7.76
Weighted-average exercise price, Ending Balance	$ 7.76	$ 0.00
Aggregate intrinsic value
Aggregate intrinsic value, Beginning Balance	$ 0
Aggregate intrinsic value, Ending Balance	$ 0	$ 0