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Stock-Based Employee Benefit Plans - Assumptions Used to Estimate Fair Value of Stock Options (Detail)
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Average expected term (years) 4 years 9 months 10 days 5 years 4 years 10 months
Expected volatility   35.30%  
Expected volatility (range), minimum 33.60%   35.30%
Expected volatility (range), maximum 35.10%   36.40%
Weighted average volatility 34.60% 35.30% 36.00%
Risk free interest rate   2.02%  
Risk free interest rate, minimum 2.03%   1.21%
Risk free interest rate, maximum 2.65%   1.49%
Average expected dividend yield 0.00% 0.07% 0.09%
Expected dividend yield (range), minimum 0.00% 0.07% 0.09%
Expected dividend yield (range), maximum 0.00% 0.07% 0.10%