Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Sep. 30, 2022 USD ($) plan	Sep. 30, 2021 USD ($)	Sep. 30, 2020	Mar. 31, 2022 USD ($)
Alternative Revenue Program [Line Items]
Number of asset management plans under management | plan	2
Composite depreciation rate for regulated property, plant and equipment	3.00%	3.00%	3.00%
Cash held in margin accounts	$ 0	$ 0
Self-insured retention	1,000,000
Commercial Paper | Five Year Unsecured Revolving Credit Agreement
Alternative Revenue Program [Line Items]
Maximum borrowing capacity	1,500,000,000			$ 1,500,000,000
Revolving Credit Facility | Five Year Unsecured Revolving Credit Agreement
Alternative Revenue Program [Line Items]
Maximum borrowing capacity	2,500,000,000
Revolving Credit Facility | $900 Million Revolving Credit Facility
Alternative Revenue Program [Line Items]
Maximum borrowing capacity	$ 900,000,000			$ 900,000,000
Pipeline and Storage
Alternative Revenue Program [Line Items]
Regulatory mechanism threshold (in percent)	75.00%
Rate case revenue benchmark	$ 69,400,000