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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025 USD ($) Units	Mar. 31, 2024 USD ($)	Mar. 31, 2025 USD ($) Units	Mar. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)
Investment Gains (Losses)
Financing Receivable, after Allowance for Credit Loss	$ 97		$ 97		$ 99
Period of time where the investor contributes capital to an investment in a private partnership - minimum | Units	3		3
Period of time where the investor contributes capital to an investment in a private partnership - maximum | Units	4		4
Minimum investment period | Units	10		10
Number of readily available quoted exchange prices for the investments	0		0
Investment funds	$ 4,917		$ 4,917		$ 4,968
NDT [Member]
Investment Gains (Losses)
Investment funds	3,300		3,300
ART [Member]
Investment Gains (Losses)
Investment funds	1,500		1,500
LTDCP [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	1	$ (1)	2	$ (2)
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	0	(2)	6	(9)
ART [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	8	(34)	28	(123)
Debt and Equity Securities, Unrealized Gain (Loss)	(6)	8	(14)	24
NDT [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(8)	(54)	(67)	(251)
Debt and Equity Securities, Unrealized Gain (Loss)	(33)	$ 24	(61)	$ 70
Real Estate Funds | NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	116		116
Real Estate Funds | ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	63		63
Private Equity Funds [Member] | NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	371		371
Private Equity Funds [Member] | ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	142		142
Private Credit [Member] | NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	86		86
Private Credit [Member] | ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 46		$ 46

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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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.

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