Regulatory Matters - Spire Alabama - Additional Information (Details) - USD ($)							1 Months Ended	12 Months Ended
	Nov. 30, 2018	Oct. 01, 2018	Feb. 01, 2018	Nov. 30, 2017	Sep. 30, 2017	Jan. 01, 2014	Nov. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Jul. 09, 2019	Oct. 02, 2018
Public Utilities, General Disclosures [Line Items]
Regulatory costs								$ 0	$ 30,600,000	$ 0
Amounts returned to customers								60,800,000	35,700,000
Amount subject to refund
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers								$ 0	2,200,000
Spire Alabama
Public Utilities, General Disclosures [Line Items]
Potential Performance-Based Adjustment, Percent								0.05%
Regulatory costs			$ 12,800,000
Rate increase (decrease), margin of index range								1.50%
Amounts returned to customers								$ 3,400,000	7,600,000
Regulatory preferred stock and debt securities issuance, amount authorized											$ 100,000,000.0
Spire Alabama | Amount subject to refund
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers								0	2,200,000
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%
Approved rate increase (decrease) amount					$ 2,700,000			$ 5,900,000	9,700,000	10,700,000
Requested rate increase	$ 8,700,000			$ 8,500,000
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%
Rate increase (decrease), margin of index range								1.75%
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											$ 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					$ 6,300,000			$ 4,200,000	$ 7,200,000	$ 6,300,000
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%