Stock-Based Compensation - Assumptions Used Estimate Fair Value of Employee Stock Purchase Plans (Detail) - $ / shares shares in Thousands	6 Months Ended				12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares purchased for offering period	1,843	3,250	2,000	2,002
Fair value per share	$ 0.96	$ 0.75	$ 0.35	$ 0.72
Assumptions
Risk-free interest rate (average)	2.29%	1.92%	0.89%	0.47%	2.90%
Weighted average expected life (years)	6 months	6 months	6 months	6 months	6 years 2 months 12 days
Volatility (average)	54.30%	81.40%	64.20%	52.60%	73.80%