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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025 USD ($) Units	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) Units	Jun. 30, 2024 USD ($)	Sep. 30, 2024 USD ($)
Investment Gains (Losses)
Financing Receivable, after Allowance for Credit Loss	$ 108		$ 108		$ 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	$ 5,277		$ 5,277		$ 4,968
NDT [Member]
Investment Gains (Losses)
Investment funds	3,500		3,500
ART [Member]
Investment Gains (Losses)
Investment funds	1,600		1,600
LTDCP [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(2)	$ 0	0	$ (2)
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	(5)	0	1	(9)
ART [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(97)	4	(69)	(119)
Debt and Equity Securities, Unrealized Gain (Loss)	10	1	5	24
NDT [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(200)	21	133	(230)
Debt and Equity Securities, Unrealized Gain (Loss)	41	$ 7	12	$ 70
Real Estate Funds \| NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	128		128
Real Estate Funds \| ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	77		77
Private Equity Funds [Member] \| NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	368		368
Private Equity Funds [Member] \| ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	143		143
Private Credit [Member] \| NDT [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	119		119
Private Credit [Member] \| ART [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 58		$ 58