Stock-Based Employee Benefit Plans - Assumptions Used to Estimate Fair Value of Stock Options (Detail)	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Average expected term (years)	4 years 2 months 4 days	4 years 13 days	4 years 1 month 6 days
Expected volatility (range), minimum	34.50%	34.60%	35.30%
Expected volatility (range), maximum	35.30%	35.60%	36.90%
Weighted average volatility	34.60%	35.10%	36.60%
Risk free interest rate, minimum	1.33%	0.76%	0.51%
Risk free interest rate, maximum	1.48%	1.16%	1.07%
Average expected dividend yield	0.14%	0.16%	0.20%
Expected dividend yield (range), minimum	0.11%	0.16%	0.18%
Expected dividend yield (range), maximum	0.14%	0.17%	0.20%