DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Open NYMEX Natural Gas Futures Positions (Details) MMBTU in Thousands	12 Months Ended
	Sep. 30, 2018 MMBTU $ / MMBTU
Fiscal 2019 | Gas Utility | Natural gas contracts | Long
Derivative [Line Items]
MMBtu (millions)	24,120
Avg. Price Per MMBtu | $ / MMBTU	2.80
Fiscal 2020 | Gas Utility | Natural gas contracts | Long
Derivative [Line Items]
MMBtu (millions)	2,680
Avg. Price Per MMBtu | $ / MMBTU	2.69
Spire Missouri
Derivative [Line Items]
MMBtu (millions)	38,400
Spire Missouri | Fiscal 2019 | Gas Utility | Natural gas contracts | Long
Derivative [Line Items]
MMBtu (millions)	24,120
Avg. Price Per MMBtu | $ / MMBTU	2.80
Spire Missouri | Fiscal 2020 | Gas Utility | Natural gas contracts | Long
Derivative [Line Items]
MMBtu (millions)	2,680
Avg. Price Per MMBtu | $ / MMBTU	2.69