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Income Taxes - Summary of Gross Amounts of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance at October 1 $ 197 $ 146 $ 134
Increase due to acquisitions 314 0 0
Increase due to current year tax positions 58 51 80
Increase due to prior year tax positions 17 9 25
Decreases due to prior year tax positions 0 0 0
Decrease due to settlements and lapse of statute of limitations (11) (9) (93)
Balance at September 30 $ 575 $ 197 $ 146