INCOME TAXES - Narrative (Details) - USD ($)	Sep. 30, 2017	Sep. 30, 2016
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	$ 478,600,000
Contribution carryforwards	12,100,000
Valuation allowance	500,000	$ 900,000
Tax credit carryforwards	2,500,000
Unrecognized tax benefits affect on the Company's effective tax rate	5,100,000	3,300,000
Income taxes accrued	0	0
income tax penalties accrued	0
Spire Missouri
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	166,000,000
Contribution carryforwards	11,200,000
Valuation allowance	500,000	900,000
Tax credit carryforwards	2,000,000
Unrecognized tax benefits affect on the Company's effective tax rate	4,800,000	3,100,000
Income taxes accrued	0	0
income tax penalties accrued	0	$ 0
Spire Alabama
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	$ 233,500,000