Fair Value Measurements - Fair Value of Long-Term Debt (Details) - USD ($) $ in Billions	Sep. 30, 2025	Sep. 30, 2024
Debt Instrument [Line Items]
Total fair value of long-term debt	$ 8.9	$ 8.3
Quoted Prices in Active Markets (Level 1)
Debt Instrument [Line Items]
Total fair value of long-term debt	8.4	8.1
Significant Other Observable Inputs (Level 2)
Debt Instrument [Line Items]
Total fair value of long-term debt	$ 0.5	$ 0.2