Regulatory Matters - Spire - Additional Information (Details) - USD ($) $ in Thousands						1 Months Ended	12 Months Ended
	Oct. 23, 2020	Aug. 28, 2020	Aug. 30, 2019	Sep. 14, 2018	Feb. 01, 2018	Dec. 31, 2015	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Public Utilities, General Disclosures [Line Items]
Regulatory costs							$ 0	$ 0	$ 30,600
Spire Gulf
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity	10.70%						10.70%
Rate increase (decrease), margin of index range							1.50%
Pro-rate rating for equity content							75.00%
Revenue variances							$ 100
Single force majeure							100
Multiple force majeures							$ 150
Recovery percentage of incurred costs							90.00%
Approved point to refund							$ 1,400
Approved rate increase (decrease) amount							1,800
Approved rate increase (decrease), annualized revenue	$ 1,100
Spire
Public Utilities, General Disclosures [Line Items]
Regulatory costs					$ 1,900
Spire Mississippi
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity								9.79%
Excess of allowed return on equity								1.00%
Return on equity shortfall								75.00%
Excess return on equity								50.00%
Recovery period						3 years
APSC | Spire
Public Utilities, General Disclosures [Line Items]
Line of credit facility borrowing capacity							50,000
Principal amount outstanding							$ 25,000
MoPSC
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease), annualized revenue		$ 600	$ 300	$ 700
Minimum [Member] | Spire Gulf
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity							10.45%
Maximum | Spire Gulf
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity							10.95%