Derivative Instruments and Hedging Activities - Schedule of Notional Amounts Related to Open Derivative Contracts (Details) € in Millions, kWh in Millions, gal in Millions, DTH in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) DTH kWh gal	Sep. 30, 2024 USD ($) DTH kWh gal	Sep. 30, 2025 EUR (€) DTH	Sep. 30, 2024 EUR (€) DTH
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 1,443	$ 1,231	€ 300	€ 300
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount	$ 500	$ 389	€ 106	€ 181
Natural Gas | Commodity contracts | Regulated Utility Operations
Derivative [Line Items]
Notional amount (in units)	28	26	28	26
Natural Gas | Natural gas futures, forward, basis swap, options and pipeline contracts | Non-utility Operations
Derivative [Line Items]
Notional amount (in units)	237	325	237	325
LPG swaps | Commodity contracts | Non-utility Operations
Derivative [Line Items]
LPG swaps (in gallons) | gal	608	458
Electricity forward and futures contracts | Commodity contracts | Non-utility Operations
Derivative [Line Items]
Electricity forward and futures contracts (in kilowatt hours) | kWh	929	1,113