Long-term Debt - Additional Information (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Long-term debt	$ 3,014,600,000	$ 2,500,000,000.0
Interest expense, net	4,400,000	5,800,000	$ 6,800,000
Spire Missouri
Debt Instrument [Line Items]
Long-term debt	1,348,000,000.0	1,098,000,000.0
Interest expense, net	200,000	800,000	$ 1,900,000
Regulatory preferred stock and debt securities issuance, amount authorized	660,000,000.0
Regulatory preferred stock and debt securities issuance, remaining authorized amount	355,000,000.0
Spire Alabama
Debt Instrument [Line Items]
Long-term debt	625,000,000.0	475,000,000.0
Interest expense, net	3,200,000	$ 1,900,000
Subsidiaries
Debt Instrument [Line Items]
Long-term debt	$ 211,600,000