INCOME TAXES - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Tax Credit Carryforward [Line Items]
Excess accumulated deferred taxes	$ 3,500,000
Federal and state loss carryforwards	711,900,000
Contribution carryforwards	12,900,000
Valuation allowance	1,400,000	$ 500,000
Tax credit carryforwards	2,400,000
Capital loss carryforward	400,000
Unrecognized tax benefits affect on the Company's effective tax rate	3,900,000	5,100,000
Income taxes accrued	0	0
Income tax penalties accrued	0	0
Spire Missouri
Tax Credit Carryforward [Line Items]
Excess accumulated deferred taxes	3,500,000
Federal and state loss carryforwards	309,200,000
Contribution carryforwards	10,900,000
Valuation allowance	1,400,000	500,000
Tax credit carryforwards	2,000,000
Unrecognized tax benefits affect on the Company's effective tax rate	3,600,000	4,800,000
Income taxes accrued	0	0
Income tax penalties accrued	0	$ 0
Spire Alabama
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	$ 342,600,000