Fair Value Measurements - Debt Securities, Trading, and Equity Securities, FV-NI (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Deferred compensation plan assets
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Equity securities, FV-NI, unrealized gain (loss)	$ 3	$ 2	$ 7	$ 5
Investments in exchange traded funds
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Equity securities, FV-NI, unrealized gain (loss)	$ 17	$ 12	$ 39	$ 23

Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(3)(d)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 321
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479536/321-10-50-4

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