Share-Based Compensation - Summary of Equity Awards Authorized and Available for Future Grant and Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 shares
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant [Roll Forward]
Available for future grants, beginning (in shares)	0
Newly authorized options (in shares)	3,800,000
Equity awards granted (in shares)	(10,200)
Cancellation and forfeitures (in shares)	0
Plan termination (in shares)	0
Available for future grants, ending (in shares)	3,789,800
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant [Roll Forward]
Available for future grants, beginning (in shares)	1,437,435
Newly authorized options (in shares)	0
Equity awards granted (in shares)	(360,999)
Cancellation and forfeitures (in shares)	0
Plan termination (in shares)	(1,076,436)
Available for future grants, ending (in shares)	0