LONG-TERM DEBT - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Long-term debt	$ 2,092,000,000
Interest expense, net	$ 2,600,000	$ 800,000	$ 200,000
Weighted average interest rate	2.40%	1.50%
Spire Missouri
Debt Instrument [Line Items]
Long-term debt	$ 880,000,000
Interest expense, net	900,000	$ 500,000	$ 200,000
Regulatory preferred stock and debt securities issuance, amount authorized	500,000,000
Spire Alabama
Debt Instrument [Line Items]
Long-term debt	325,000,000
Subsidiaries
Debt Instrument [Line Items]
Long-term debt	$ 72,000,000