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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended
	Dec. 31, 2023 USD ($) Units	Dec. 31, 2022 USD ($)	Sep. 30, 2023 USD ($)
Investment Gains (Losses)
Financing Receivable, after Allowance for Credit Loss	$ 97		$ 96
Period of time where the investor contributes capital to an investment in a private partnership - minimum | Units	3
Period of time where the investor contributes capital to an investment in a private partnership - maximum | Units	4
Minimum investment period | Units	10
Number of readily available quoted exchange prices for the investments	0
Investment funds	$ 4,472		$ 4,123
NDT [Member]
Investment Gains (Losses)
Investment funds	3,000
ART [Member]
Investment Gains (Losses)
Investment funds	1,300
LTDCP [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(1)	$ 0
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	(7)	(3)
ART [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(89)	(69)
NDT [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(197)	$ (136)
Real Estate Funds | NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	110
Real Estate Funds | ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	54
Private Equity Funds [Member] | NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	296
Private Equity Funds [Member] | ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	122
Private Credit [Member] | NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	88
Private Credit [Member] | ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 47

X

- References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-Publisher FASB
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Number of readily available quoted exchange prices for the investments

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Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
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-Section S99
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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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Amount of unrealized gain (loss) on investment.

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