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Income Taxes - Summary of Gross Amounts of Unrecognized Tax Benefits (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2012
Income Tax Disclosure [Abstract]      
Unrecognized tax benefits, beginning balance $ 146 $ 134 $ 117
Increase due to current year tax positions 51 80 37
Increase due to prior year tax positions 9 25 2
Decreases due to prior year tax positions     (3)
Decrease due to settlements and lapse of statute of limitations (9) (93) (19)
Unrecognized tax benefits, ending balance $ 197 $ 146 $ 134