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Employee Benefits - Black-Scholes Option Pricing Model to Determine Grant-date Fair Value for Stock Options (Detail)
12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Risk-free interest rates 0.88% 1.24% 1.91%
Dividend yield 4.10% 3.42% 3.74%
Life of option 6 years 2 months 23 days 6 years 2 months 19 days 5 years 11 months 5 days
Share price volatility 36.00% 39.00% 34.00%
Forfeitures (by employee group) Minimum 0.00% 0.00% 0.00%
Forfeitures (by employee group) Maximum 10.00% 10.00% 10.00%