DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Exchange-Traded/Cleared Derivatives (Details) MMBTU in Thousands	12 Months Ended
	Sep. 30, 2017 MMBTU $ / MMBTU $ / gal
Spire Missouri
Derivative [Line Items]
MMBtu (millions)	33,900
Avg. Price Per MMBtu | $ / gal	1.27
Natural gas contracts | Fiscal 2021 | Spire Missouri
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Natural gas contracts | Fiscal 2021 | Spire marketing
Derivative [Line Items]
MMBtu (millions)	280
Avg. Price Per MMBtu | $ / MMBTU	2.90
Natural gas contracts | Fiscal 2022 | Spire Missouri
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Natural gas contracts | Fiscal 2022 | Spire marketing
Derivative [Line Items]
MMBtu (millions)	220
Avg. Price Per MMBtu | $ / MMBTU	3.00
Short | Gas Utility: | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility: | Natural gas contracts | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility: | ICE open daily swap positions [Member] | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility: | ICE Open basis swap positions | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility: | ICE Open basis swap positions | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility: | ICE Open basis swap positions | Fiscal 2020
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Marketing: | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	8,660
Avg. Price Per MMBtu | $ / MMBTU	3.34
Short | Gas Marketing: | Natural gas contracts | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	2,070
Avg. Price Per MMBtu | $ / MMBTU	3.13
Short | Gas Marketing: | ICE open daily swap positions [Member] | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	1,320
Avg. Price Per MMBtu | $ / MMBTU	2.87
Short | Gas Marketing: | ICE Open basis swap positions | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	14,040
Avg. Price Per MMBtu | $ / MMBTU	0.10
Short | Gas Marketing: | ICE Open basis swap positions | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	3,350
Avg. Price Per MMBtu | $ / MMBTU	0.27
Short | Gas Marketing: | ICE Open basis swap positions | Fiscal 2020
Derivative [Line Items]
MMBtu (millions)	310
Avg. Price Per MMBtu | $ / MMBTU	0.36
Long | Gas Utility: | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	14,180
Avg. Price Per MMBtu | $ / MMBTU	2.98
Long | Gas Utility: | Natural gas contracts | Fiscal 2018 | Spire Missouri
Derivative [Line Items]
MMBtu (millions)	14,180
Avg. Price Per MMBtu | $ / MMBTU	2.98
Long | Gas Utility: | Natural gas contracts | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	1,680
Avg. Price Per MMBtu | $ / MMBTU	2.89
Long | Gas Utility: | Natural gas contracts | Fiscal 2019 | Spire Missouri
Derivative [Line Items]
MMBtu (millions)	1,680
Avg. Price Per MMBtu | $ / MMBTU	2.89
Long | Gas Utility: | Natural gas contracts | Fiscal 2020
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Long | Gas Utility: | ICE open daily swap positions [Member] | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Long | Gas Utility: | ICE Open basis swap positions | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Long | Gas Utility: | ICE Open basis swap positions | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Long | Gas Utility: | ICE Open basis swap positions | Fiscal 2020
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Long | Gas Marketing: | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	3,780
Avg. Price Per MMBtu | $ / MMBTU	3.16
Long | Gas Marketing: | Natural gas contracts | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	1,840
Avg. Price Per MMBtu | $ / MMBTU	3.02
Long | Gas Marketing: | Natural gas contracts | Fiscal 2020
Derivative [Line Items]
MMBtu (millions)	660
Avg. Price Per MMBtu | $ / MMBTU	2.91
Long | Gas Marketing: | ICE open daily swap positions [Member] | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	780
Avg. Price Per MMBtu | $ / MMBTU	2.79
Long | Gas Marketing: | ICE Open basis swap positions | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	11,120
Avg. Price Per MMBtu | $ / MMBTU	0.38
Long | Gas Marketing: | ICE Open basis swap positions | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	4,510
Avg. Price Per MMBtu | $ / MMBTU	0.45
Long | Gas Marketing: | ICE Open basis swap positions | Fiscal 2020
Derivative [Line Items]
MMBtu (millions)	620
Avg. Price Per MMBtu | $ / MMBTU	0.45