Income Taxes - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
Charitable contribution carryforwards			$ 6,900
Remeasured federal effects			500
Unrecognized tax benefits, period increase			30,900
Interest and penalties			700	$ 2,200	$ 1,600
Payment of interest and penalties accrued	$ 6,100		8,200	7,900	6,100
Reduction in deferred tax liabilities		$ 32,500
Regulatory liability			1,367,013	1,386,126
Income tax benefit			20,962	0	0
One-time income tax benefit			0	0	$ 158,782
Deferred tax liability, regulated operations			$ 706,700	$ 726,300
Statutory federal income tax rate	35.00%		21.00%	21.00%	24.50%
Domestic Tax Authority
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards			$ 446,900
Federal alternative minimum tax credit carryforwards			10,100
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards			29,600
Federal alternative minimum tax credit carryforwards			1,800
Federal effects			7,900
Valuation Allowance, Tax Credit Carryforward
Income Tax Contingency [Line Items]
Valuation allowance			$ 1,100
Minimum
Income Tax Contingency [Line Items]
Recognition period			15 years
Maximum
Income Tax Contingency [Line Items]
Recognition period			46 years
Deferred Income Tax Charge - Remeasurement Due to Law Change under the Reg Liability [Member]
Income Tax Contingency [Line Items]
Regulatory liability		$ 12,100