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Weighted Average Actuarial Assumptions (Detail)
 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected long-term rate of return on plan assets [1] 7.00% 7.00% 7.00%
Rate of increase in compensation levels for:      
Benefit obligation 4.00% 4.00% 4.50%
Net periodic benefit cost 4.00% 4.50% 4.50%
Pension Benefits      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate for benefit obligation [2] 4.33% 4.27% 4.81%
Discount rate for net periodic benefit cost 4.27% 4.81% 3.85%
Other Postretirement Benefits      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate for benefit obligation [2] 2.43% 2.59% 2.76%
Discount rate for net periodic benefit cost 2.59% 2.76% 2.21%
[1] Primarily based on the targeted allocation, and evaluated for reasonableness by considering such factors as: (i) actual return on plan assets; (ii) historical rates of return on various asset classes in the portfolio; (iii) projections of returns on various asset classes; and (iv) current and prospective capital market conditions and economic forecasts.
[2] Based on a “bond duration matching” methodology, which reflects the matching of projected plan liability cash flows to an average of high-quality corporate bond yield curves whose duration matches the projected cash flows.