Unaudited Financial Information - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,362,984		$ 1,367,013
Effective income tax rate	18.80%	19.20%
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 694,018		718,651
Regulatory excess deferred taxes, to be returned, tranche one
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 243,000
Regulatory excess deferred taxes, to be returned, tranche one | Minimum
Regulatory Liabilities [Line Items]
Return basis, term	35 months
Regulatory excess deferred taxes, to be returned, tranche one | Maximum
Regulatory Liabilities [Line Items]
Return basis, term	60 months
Regulatory excess deferred taxes, to be returned, tranche two
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 430,100
Regulatory excess deferred taxes, to be returned, tranche two | Minimum
Regulatory Liabilities [Line Items]
Return basis, term	15 years
Regulatory excess deferred taxes, to be returned, tranche two | Maximum
Regulatory Liabilities [Line Items]
Return basis, term	69 years
Regulatory excess deferred taxes, to be returned, tranche three
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 12,100
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 106,300		$ 20,900