Long-term Debt - Additional Information (Details) - USD ($)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Mar. 24, 2020
Debt Instrument [Line Items]
Long-term debt	$ 2,500,000,000.0	$ 2,137,000,000.0
Interest expense, net	5,800,000	6,800,000	$ 2,600,000
Spire Missouri
Debt Instrument [Line Items]
Long-term debt	1,098,000,000.0	930,000,000.0
Interest expense, net	800,000	1,900,000	$ 900,000
Regulatory preferred stock and debt securities issuance, amount authorized	500,000,000.0
Regulatory preferred stock and debt securities issuance, remaining authorized amount	125,000,000.0
Spire Alabama
Debt Instrument [Line Items]
Long-term debt	475,000,000.0	$ 415,000,000.0
Interest expense, net	1,900,000
Regulatory preferred stock and debt securities issuance, amount authorized				$ 150,000,000.0
Subsidiaries
Debt Instrument [Line Items]
Long-term debt	$ 237,000,000.0