Investment Income, Net (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Interest and dividend income on cash and investments	$ 173	$ 92	$ 75
Gain on other investments		6	5
Investment securities, trading:
Unrealized gains, net	2	6	3
Realized gains, net	4	2	0
Investment securities, available-for-sale:
Realized gains (losses), net from sales	98	(1)	3
Other-than-temporary impairment on investments			(4)
Investment income	470	$ 105	82
Donation [Member]
Investment securities, available-for-sale:
Realized gains (losses), net from sales	$ 193		$ 0

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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Amount of operating dividend and interest income, including amortization and accretion of premiums and discounts, on securities.

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Amount after accretion (amortization) of discount (premium), and investment expense, of interest income and dividend income on nonoperating securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
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Other-than-temporary impairment losses realized

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