Regulatory Matters - Spire Alabama - Additional Information (Details) - USD ($)				1 Months Ended	12 Months Ended
	Dec. 01, 2019	Oct. 01, 2018	Jan. 01, 2014	Nov. 30, 2019	Sep. 30, 2020	Sep. 30, 2018	Mar. 24, 2020	Dec. 02, 2019	Sep. 30, 2019	Jan. 15, 2019
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers					$ 113,000,000.0				$ 60,800,000
Spire Alabama
Public Utilities, General Disclosures [Line Items]
Potential Performance-Based Adjustment, Percent					0.05%
Rate increase (decrease), margin of index range					1.50%
Amounts returned to customers					$ 3,900,000				$ 3,400,000
Regulatory preferred stock and debt securities issuance, amount authorized							$ 150,000,000.0
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]
Approved return rate on equity		10.40%	10.80%
Maximum return on equity, percentage of prior year revenues					4.00%
Requested rate increase	$ 5,900,000
Return on equity threshold limit exceeded					10.50%
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%
Gas supply adjustment decrease on annual basis					$ 13,900,000
Rate increase (decrease), margin of index range									1.75%
Approved rate increase (decrease) amount					5,900,000	$ 9,700,000
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
APSC | Spire Alabama | Amount subject to refund
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers						$ 7,200,000			$ 4,200,000
AIM | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) on allowed return on equity, percentage					0.10%
Minimum [Member] | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity		10.15%	10.50%
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%	10.95%
Equity limitation as percent of capitalization			56.50%