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Components of Net Periodic Benefit Cost (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Mar. 31, 2014
Mar. 31, 2013
Pension Benefits
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 12 $ 12 $ 23 $ 22
Interest cost 11 9 21 18
Expected return on assets (17) (15) (34) (31)
Amortization of:        
Prior service credit (2) (3) (4) (5)
Actuarial loss    7    14
Net benefit cost 4 10 6 18
Curtailment gain (3)    (3)   
Settlement loss       1   
Total net periodic benefit cost 1 10 4 18
Other Postretirement Benefits
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost            
Interest cost            
Expected return on assets            
Amortization of:        
Prior service credit (1)    (2) (1)
Actuarial loss            
Net benefit cost (1) 0 (2) (1)
Curtailment gain            
Settlement loss            
Total net periodic benefit cost $ (1)    $ (2) $ (1)