Share Based Payments - Summary of Non-Employee Stock Options Required to be Accounted for As Liabilities (Detail) - Non-employee Stock Options - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of non-employee stock options at beginning of period	$ 521	$ 1,617
Reallocation of vested non-employee stock options	268
Exercised		(156)
Cancelled		(294)
Change in fair value of non-employee stock options during the period	828	1,470
Fair value of non-employee stock options at end of period	$ 1,617	$ 2,637