FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2015	Sep. 30, 2014
Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	$ (1.7)	$ (2.5)
Derivative asset	20.0	19.6
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(9.9)	(5.5)
Derivative liability	7.1	4.0
Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	17.7	18.1
LIABILITIES
Derivative liability before netting adjustment	9.9	5.4
Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	4.0
LIABILITIES
Derivative liability before netting adjustment	7.1	4.1
Gas Utility | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(4.5)
Derivative asset		22.7
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(13.7)	(7.5)
Derivative liability	7.5	4.5
Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		19.1
LIABILITIES
Derivative liability before netting adjustment	10.3	6.5
Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		7.9
LIABILITIES
Derivative liability before netting adjustment	10.9	5.5
Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.2
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(2.5)
Derivative asset	20.0	19.6
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(9.9)	(5.5)
Derivative liability	7.1	4.0
Laclede Gas | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	17.7	18.1
LIABILITIES
Derivative liability before netting adjustment	9.9	5.4
Laclede Gas | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	4.0
LIABILITIES
Derivative liability before netting adjustment	7.1	4.1
Laclede Gas | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
U. S. stock/bond mutual funds | Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0
Derivative asset	20.0	19.6
U. S. stock/bond mutual funds | Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	16.0	15.7
U. S. stock/bond mutual funds | Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	3.9
U. S. stock/bond mutual funds | Gas Utility | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
U. S. stock/bond mutual funds | Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(6.3)
Derivative asset	23.8
U. S. stock/bond mutual funds | Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	20.4
U. S. stock/bond mutual funds | Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	9.0
U. S. stock/bond mutual funds | Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.7
U. S. stock/bond mutual funds | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0
Derivative asset	20.0	19.6
U. S. stock/bond mutual funds | Laclede Gas | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	16.0	15.7
U. S. stock/bond mutual funds | Laclede Gas | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	3.9
U. S. stock/bond mutual funds | Laclede Gas | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
Natural gas commodity contracts | Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(0.2)
Derivative asset	2.6	2.7
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(0.2)
Derivative liability	0.4	0.5
Natural gas commodity contracts | Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Natural gas commodity contracts | Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	2.4	2.7
LIABILITIES
Derivative liability before netting adjustment	0.9	0.7
Natural gas commodity contracts | Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.7	0.2
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX/ICE natural gas contracts | Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(2.4)
Derivative asset	0.0	0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(9.5)	(5.2)
Derivative liability	0.0	0.0
NYMEX/ICE natural gas contracts | Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	1.7	2.4
LIABILITIES
Derivative liability before netting adjustment	9.5	5.2
NYMEX/ICE natural gas contracts | Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX/ICE natural gas contracts | Gas Utility | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX/ICE natural gas contracts | Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(4.1)	(1.8)
Derivative asset	1.2	0.4
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(3.3)	(1.8)
Derivative liability	0.0	0.0
NYMEX/ICE natural gas contracts | Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	2.7	1.0
LIABILITIES
Derivative liability before netting adjustment	0.4	1.1
NYMEX/ICE natural gas contracts | Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	2.6	1.2
LIABILITIES
Derivative liability before netting adjustment	2.9	0.7
NYMEX/ICE natural gas contracts | Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX/ICE natural gas contracts | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(2.4)
Derivative asset	0.0	0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(9.5)	(5.2)
Derivative liability	0.0	0.0
NYMEX/ICE natural gas contracts | Laclede Gas | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	1.7	2.4
LIABILITIES
Derivative liability before netting adjustment	9.5	5.2
NYMEX/ICE natural gas contracts | Laclede Gas | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX/ICE natural gas contracts | Laclede Gas | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
OTCBB natural gas contracts | Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(0.1)
Derivative asset		0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0	(0.1)
Derivative liability	7.1	4.0
OTCBB natural gas contracts | Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
OTCBB natural gas contracts | Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		0.1
LIABILITIES
Derivative liability before netting adjustment	7.1	4.1
OTCBB natural gas contracts | Gas Utility | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
OTCBB natural gas contracts | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(0.1)
Derivative asset		0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0	(0.1)
Derivative liability	7.1	4.0
OTCBB natural gas contracts | Laclede Gas | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
OTCBB natural gas contracts | Laclede Gas | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		0.1
LIABILITIES
Derivative liability before netting adjustment	7.1	4.1
OTCBB natural gas contracts | Laclede Gas | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX gasoline and heating oil contracts | Gas Utility
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.4)
Derivative liability	0.0
NYMEX gasoline and heating oil contracts | Gas Utility | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	0.4
NYMEX gasoline and heating oil contracts | Gas Utility | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	0.0
NYMEX gasoline and heating oil contracts | Gas Utility | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0
NYMEX gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		(0.2)
Derivative liability		0.0
NYMEX gasoline and heating oil contracts | Laclede Gas | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment		0.2
NYMEX gasoline and heating oil contracts | Laclede Gas | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment		0.0
NYMEX gasoline and heating oil contracts | Laclede Gas | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment		0.0
Gasoline and heating oil contracts | Gas Utility
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		(0.2)
Derivative liability		0.0
Gasoline and heating oil contracts | Gas Utility | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment		0.2
Gasoline and heating oil contracts | Gas Utility | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment		0.0
Gasoline and heating oil contracts | Gas Utility | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment		$ 0.0
Gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.4)
Derivative liability	0.0
Gasoline and heating oil contracts | Laclede Gas | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	0.4
Gasoline and heating oil contracts | Laclede Gas | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	0.0
Gasoline and heating oil contracts | Laclede Gas | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	$ 0.0