Regulation - Narrative (Details) - USD ($) $ in Thousands	Oct. 01, 2023	Mar. 23, 2023	Sep. 01, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Regulatory Assets [Line Items]
Regulatory assets				$ 554,928	$ 2,605,372
Winter Storm Uri incremental costs
Regulatory Assets [Line Items]
Regulatory assets				32,115	$ 2,109,454
Kansas | Winter Storm Uri
Regulatory Assets [Line Items]
Regulatory assets						$ 92,300
Texas | Winter Storm Uri
Regulatory Assets [Line Items]
Regulatory assets						$ 2,020,000
Texas | Extraordinary Gas Cost | Winter Storm Uri
Regulatory Assets [Line Items]
Regulatory asset, authorized		$ 3,500,000
Regulatory asset, proceeds from customer rate relief bonds		$ 2,020,000
Texas | Extraordinary Gas Cost | Winter Storm Uri | Minimum
Regulatory Assets [Line Items]
Regulatory asset, maturity period		12 years
Texas | Extraordinary Gas Cost | Winter Storm Uri | Maximum
Regulatory Assets [Line Items]
Regulatory asset, maturity period		18 years
Texas | Winter Storm Uri incremental costs | Winter Storm Uri
Regulatory Assets [Line Items]
Regulatory asset, deferred costs			$ 32,400
Regulatory asset, deferred costs, remaining				$ 10,900
Texas | Winter Storm Uri incremental costs | Winter Storm Uri | Subsequent Event
Regulatory Assets [Line Items]
Regulatory asset, recovering cost	$ 21,200
Regulatory asset, recovering costs period	12 months