DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Exchange-Traded/Cleared Derivatives (Details) MMBTU in Thousands	12 Months Ended
	Sep. 30, 2016 MMBTU $ / gal $ / MMBTU
Laclede Gas
Derivative [Line Items]
MMBtu (millions)	19,800
Avg. Price Per MMBtu | $ / gal	1.37
Short | Gas Utility | Natural gas contracts | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Short | Gas Utility | ICE Open basis swap positions | Fiscal 2017
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 Marketing | Natural gas contracts | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	12,200
Avg. Price Per MMBtu | $ / MMBTU	2.86
Short | Gas Marketing | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	1,270
Avg. Price Per MMBtu | $ / MMBTU	2.95
Short | Gas Marketing | ICE Open basis swap positions | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	11,880
Avg. Price Per MMBtu | $ / MMBTU	0.19
Short | Gas Marketing | ICE Open basis swap positions | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	930
Avg. Price Per MMBtu | $ / MMBTU	0.27
Short | Gas Marketing | ICE Open basis swap positions | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	2,270
Avg. Price Per MMBtu | $ / MMBTU	0.13
Long | Gas Utility | Natural gas contracts | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	18,430
Avg. Price Per MMBtu | $ / MMBTU	2.95
Long | Gas Utility | Natural gas contracts | Fiscal 2017 | Laclede Gas
Derivative [Line Items]
MMBtu (millions)	18,430
Avg. Price Per MMBtu | $ / MMBTU	2.95
Long | Gas Utility | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	330
Avg. Price Per MMBtu | $ / MMBTU	2.83
Long | Gas Utility | Natural gas contracts | Fiscal 2018 | Laclede Gas
Derivative [Line Items]
MMBtu (millions)	330
Avg. Price Per MMBtu | $ / MMBTU	2.83
Long | Gas Utility | Natural gas contracts | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00
Long | Gas Utility | ICE Open basis swap positions | Fiscal 2017
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 | OTC Open long futures/swap positions | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	320
Avg. Price Per MMBtu | $ / MMBTU	3.64
Long | Gas Utility | OTC Open long futures/swap positions | Fiscal 2017 | Laclede Gas
Derivative [Line Items]
MMBtu (millions)	320
Avg. Price Per MMBtu | $ / MMBTU	3.64
Long | Gas Marketing | Natural gas contracts | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	4,620
Avg. Price Per MMBtu | $ / MMBTU	3.12
Long | Gas Marketing | Natural gas contracts | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	450
Avg. Price Per MMBtu | $ / MMBTU	3.02
Long | Gas Marketing | Natural gas contracts | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	40
Avg. Price Per MMBtu | $ / MMBTU	2.89
Long | Gas Marketing | ICE Open basis swap positions | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	16,620
Avg. Price Per MMBtu | $ / MMBTU	0.34
Long | Gas Marketing | ICE Open basis swap positions | Fiscal 2018
Derivative [Line Items]
MMBtu (millions)	2,010
Avg. Price Per MMBtu | $ / MMBTU	0.43
Long | Gas Marketing | ICE Open basis swap positions | Fiscal 2019
Derivative [Line Items]
MMBtu (millions)	80
Avg. Price Per MMBtu | $ / MMBTU	0.40
Long | Gas Marketing | OTC Open long futures/swap positions | Fiscal 2017
Derivative [Line Items]
MMBtu (millions)	0
Avg. Price Per MMBtu | $ / MMBTU	0.00