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Employee Stock Purchase Plan - Weighted Average Assumptions of Employee Stock Purchase Plan (Detail) - Stock Purchase Plan [Member]
3 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Dividend yield 0.00% 0.00%
Risk-free interest rate 2.40% 1.20%
Volatility 52.20% 50.90%
Expected life 6 months 6 months