Stock-Based Employee Benefit Plans - Assumptions Used to Estimate Fair Value of Stock Options (Detail)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Average expected term (years)	4 years 10 months	4 years 2 months 4 days	4 years 13 days
Expected volatility (range), minimum	35.30%	34.50%	34.60%
Expected volatility (range), maximum	36.40%	35.30%	35.60%
Weighted average volatility	36.00%	34.60%	35.10%
Risk free interest rate, minimum	1.21%	1.33%	0.76%
Risk free interest rate, maximum	1.49%	1.48%	1.16%
Average expected dividend yield	0.09%	0.14%	0.16%
Expected dividend yield (range), minimum	0.09%	0.11%	0.16%
Expected dividend yield (range), maximum	0.10%	0.14%	0.17%