Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Contingency [Line Items]
Statutory federal income tax rate	21.00%	24.50%	35.00%
Deferred tax liability, regulated operations	$ 726,300	$ 744,900
One-time income tax benefit		(158,800)
Regulatory liability	1,386,126	1,510,997
Charitable contribution carryforwards	5,500
Remeasured federal effects	500
Interest and penalties	2,200	1,600	$ 1,100
Payment of interest and penalties accrued	7,900	6,100	4,500
Unrecognized tax benefits - ending balance	27,716	26,203	$ 23,719
Regulatory Cost of Service Liability	5,200	22,500
Domestic Tax Authority
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	451,800
Federal alternative minimum tax credit carryforwards	10,100
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards	33,300
Federal alternative minimum tax credit carryforwards	1,800
Federal effects	$ 8,800
Valuation Allowance, Tax Credit Carryforward
Income Tax Contingency [Line Items]
Valuation allowance		1,800
Minimum
Income Tax Contingency [Line Items]
Recognition period	13 years
Maximum
Income Tax Contingency [Line Items]
Recognition period	51 years
Regulatory Cost of Service Reserve
Income Tax Contingency [Line Items]
Regulatory liability	$ 5,238	$ 22,508