Fair Value Measurements and Investments - Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Interest and dividend income on cash and investments	$ 247	$ 173	$ 92
Realized gains (losses), net on debt securities
Realized gains (losses), net	1	0	(1)
Equity securities:
Unrealized gains (losses), net	117	2	6
Investment income	383	470	105
Donation
Equity securities:
Realized gains (losses), net	0	193	0
Excluding Donation
Equity securities:
Realized gains (losses), net	$ 18	$ 102	$ 8

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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