Derivative Instruments and Hedging Activities - Open NYMEX Natural Gas Futures Positions (Details) - Gas Utility - Natural Gas Contracts - Long MMBTU in Thousands	12 Months Ended
	Sep. 30, 2019 MMBTU $ / MMBTU
Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	66,360
Avg. Price Per MMBtu | $ / MMBTU	2.66
Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	2,680
Avg. Price Per MMBtu | $ / MMBTU	2.68
Spire Missouri | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	66,360
Avg. Price Per MMBtu | $ / MMBTU	2.66
Spire Missouri | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	2,680
Avg. Price Per MMBtu | $ / MMBTU	2.68