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Stock-Based Compensation - Assumptions Used to Compute Share-Based Compensation Costs for Stock Options Granted (Detail) - $ / shares
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]          
Weighted average grant date fair value of stock options         $ 1.56
Assumptions          
Risk-free interest rate (weighted 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 (weighted average) 54.30% 81.40% 64.20% 52.60% 73.80%
Forfeiture rate         26.60%