Subsequent Event - Additional Information (Details) - Infrastructure System Replacement Surcharge - Spire Missouri - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Sep. 30, 2018	Apr. 30, 2018	Sep. 30, 2019
Subsequent Event [Line Items]
Estimated regulatory liabilities			$ 12.2
Refund ordered amounts collected prior to last rate case		$ 4.2
Annual authorized revenue			12.4
Estimated amount earned			4.6
MoPSC
Subsequent Event [Line Items]
Approved estimate revenue	$ 8.0
After-tax impact on loss contingency provision reducing net income			$ (9.3)
After-tax impact of provision to decrease net income per diluted share			$ 0.18