DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES Narrative (Details) gal in Millions, MMBTU in Millions				3 Months Ended			12 Months Ended
	Jun. 03, 2016 USD ($)	May 21, 2015 USD ($)	Aug. 06, 2014 USD ($)	Sep. 30, 2016 USD ($) MMBTU $ / gal gal	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2016 USD ($) MMBTU $ / gal gal	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	May 05, 2016 USD ($)	Dec. 01, 2015 USD ($)	Sep. 15, 2015 USD ($)	Apr. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Aug. 19, 2014 USD ($)	Jul. 08, 2014 USD ($)
Derivative [Line Items]
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) | MMBTU				122.7			122.7
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2016 | MMBTU				110.8			110.8
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2017 | MMBTU				11.4			11.4
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2018 | MMBTU				0.2			0.2
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2019 | MMBTU				0.3			0.3
Notional amount					$ 120,000,000.0
Long-term debt issued amount															$ 625,000,000.0
Term of contract					7 years
Derivative average fixed interest rate					1.501%
Unrealized gain on energy-related derivatives							$ 100,000	$ 2,800,000	$ 1,600,000
Gain (loss) to be reclassified within twelve months							0
Cash margin receivables not offset with derivatives				$ 2,900,000			$ 2,900,000	5,900,000
Alagasco
Derivative [Line Items]
Unsecured debt														$ 115,000,000
Gasoline futures contracts notional amount (in gallons) | gal				0.6			0.6
Average price of future contracts (dollars per gallon) | $ / gal				1.53			1.53
Alagasco | 3.21% Notes, due September 15, 2025
Derivative [Line Items]
Unsecured debt												$ 35,000,000
Alagasco | Notes 4.31%, Due December 1, 2045
Derivative [Line Items]
Unsecured debt											$ 80,000,000
Laclede Gas
Derivative [Line Items]
MMBtu (millions) | MMBTU							19.8
Gain (loss) to be reclassified within twelve months							$ 0
Cash margin receivables not offset with derivatives				$ 500,000			$ 500,000	$ 5,900,000
Gasoline futures contracts notional amount (in gallons) | gal				1.8			1.8
Average price of future contracts (dollars per gallon) | $ / gal				1.37			1.37
Interest rate swaps
Derivative [Line Items]
Notional amount						$ 105,000,000
Derivative fixed interest rate						1.776%
Term of contract						5 years
Unrealized gain on energy-related derivatives				$ 1,000,000			$ (3,000,000)
Designated as Hedging Instrument | Interest rate swaps | Alagasco
Derivative [Line Items]
Gain on settlement of derivative		$ 2,700,000
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps
Derivative [Line Items]
Notional amount										$ 85,000,000.0			$ 24,000,000.0			$ 375,000,000.0
Loss on termination and settlement of interest rate swap			$ 19,000,000
Effective portion of gain (loss) recognized in OCI on derivatives	$ (400,000)
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Alagasco
Derivative [Line Items]
Notional amount													$ 24,000,000.0	$ 80,500,000
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Alagasco | Minimum
Derivative [Line Items]
Derivative fixed interest rate														2.18%
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Alagasco | Maximum
Derivative [Line Items]
Derivative fixed interest rate														2.85%