v3.3.0.814
Plan's Investments at Fair Value (Detail) - USD ($)
$ in Millions
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount $ 0    
Pension Benefits      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 1,022 $ 1,117 $ 1,055
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount 0 0  
Fair Value, Measurements, Recurring | Level 1      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 756 842  
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 11 22  
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements [1] 74 53  
Fair Value, Measurements, Recurring | Level 1 | Equity securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 671 767  
Fair Value, Measurements, Recurring | Level 2      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 235 250  
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 169 144  
Fair Value, Measurements, Recurring | Level 2 | U.S. government-sponsored debt securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements [1] 66 106  
Fair Value, Measurements, Recurring | Level 3      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 31 25  
Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 31 25  
Total | Fair Value, Measurements, Recurring      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 1,022 1,117  
Total | Fair Value, Measurements, Recurring | Cash equivalents      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 11 22  
Total | Fair Value, Measurements, Recurring | Corporate debt securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 169 144  
Total | Fair Value, Measurements, Recurring | U.S. government-sponsored debt securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements [1] 66 106  
Total | Fair Value, Measurements, Recurring | U.S. Treasury securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements [1] 74 53  
Total | Fair Value, Measurements, Recurring | Asset-backed securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements 31 25  
Total | Fair Value, Measurements, Recurring | Equity securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value Measurements $ 671 $ 767  
[1] U.S. Treasury securities are Level 1 assets, but were previously presented in aggregate with U.S. government-sponsored debt securities as Level 2. The Company now presents U.S. Treasury securities separately for disclosure purposes. There were no changes to the valuation techniques or related inputs used to measure fair value. Prior period amounts reflect the change in presentation.