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, 2019	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares purchased for offering period	2,195	2,112	1,843	3,250
Fair value per share	$ 2.28	$ 1.07	$ 0.96	$ 0.75
Assumptions
Risk-free interest rate (average)	1.84%	2.44%	2.29%	1.92%	2.36%	2.90%
Weighted average expected life (years)	6 months	6 months	6 months	6 months	6 years 2 months 12 days	6 years 2 months 12 days
Volatility (average)	86.30%	51.70%	54.30%	81.40%	78.70%	73.80%