Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Beginning balance	$ 288	$ 193	$ 596
Additions for tax positions related to the current year	108	110	76
Additions for tax positions of prior years	45	12	5
Reductions for tax positions of prior years	(22)	(21)	(471)
Settlements with taxing authorities	0	0	(7)
Statute closings	(5)	(6)	(6)
Ending balance	$ 414	$ 288	$ 193

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Amount of unrecognized tax benefits.

+ References

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-Name Accounting Standards Codification
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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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