Regulatory Matters - Spire Alabama - Additional Information (Details) - USD ($)			1 Months Ended	12 Months Ended
	Dec. 01, 2020	Oct. 01, 2018	Nov. 30, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Dec. 02, 2019	Jan. 15, 2019
Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved point of test giveback				$ 2,200,000
Rate increase (decrease), margin of index range				1.50%
APSC | ESR
Public Utilities, General Disclosures [Line Items]
Extraordinary O&M expenses in excess of				$ 1,000,000.0
Single force majeure				300,000
Multiple force majeures				400,000
Revenue variances				$ 400,000
ESR term				9 years
Underfunded ESR balances amortization term				5 years
ESR annual limitation				$ 700,000
APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Maximum return on equity, percentage of prior year revenues				4.00%
Approved return rate on equity		10.40%
Requested rate increase	$ 8,300,000
Approved percentage of excess natural gas supply and capacity retain			25.00%
Approved percentage of excess natural gas supply and capacity to customers			75.00%
Approved rate increase (decrease) amount				$ 8,800,000	$ 5,200,000	$ 5,900,000
Line of credit facility borrowing capacity				200,000,000.0
Principal amount outstanding				25,000,000.0
Regulatory preferred stock and debt securities issuance, amount authorized				$ 150,000,000.0			$ 100,000,000.0	$ 90,000,000.0
Minimum [Member] | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity		10.15%
Equity limitation as percent of capitalization		55.50%
Maximum | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity		10.65%
Equity limitation as percent of capitalization		56.50%