Income Taxes - Narrative (Details) - USD ($)		3 Months Ended	12 Months Ended
	Dec. 22, 2017	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Contingency [Line Items]
Statutory federal income tax rate			24.50%	35.00%	35.00%
Deferred tax liability	$ 905,300,000
Deferred tax liability, regulated operations	$ 746,500,000		$ 744,900,000
One-time income tax benefit			(158,782,000)	$ 0	$ 0
Regulatory liability			1,510,997,000	555,657,000
Charitable contribution carryforwards			5,300,000
Remeasured federal effects			400,000
Reduction in valuation allowance		$ 4,200,000
Interest and penalties			1,600,000	1,100,000	2,500,000
Payment of interest and penalties accrued			6,100,000	4,500,000	$ 3,300,000
Domestic Tax Authority
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards			430,000,000
Federal alternative minimum tax credit carryforwards			10,100,000
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Federal net operating loss carryforwards			31,400,000
Federal alternative minimum tax credit carryforwards			1,500,000
Federal effects			8,400,000
Valuation Allowance, Tax Credit Carryforward
Income Tax Contingency [Line Items]
Valuation allowance			$ 0	1,500,000
Minimum
Income Tax Contingency [Line Items]
Recognition period			18 years
Maximum
Income Tax Contingency [Line Items]
Recognition period			40 years
Regulatory Cost of Service Reserve
Income Tax Contingency [Line Items]
Regulatory liability			$ 22,508,000	$ 0