FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2016	Sep. 30, 2015	Mar. 31, 2015
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	$ (20.0)
Derivative liability	10.7
Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	$ (1.3)	$ (0.6)
Derivative asset	20.2	19.5	20.3
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(13.7)	(16.7)	(15.1)
Derivative liability	6.0	5.9	14.5
Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	15.8
Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	16.2	16.8	17.0
LIABILITIES
Derivative liability before netting adjustment	13.7	16.7	15.1
Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	14.9
Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	4.0	4.0	3.9
LIABILITIES
Derivative liability before netting adjustment	6.0	5.9	14.5
Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0
Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
U. S. stock/bond mutual funds | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	20.2	19.5	20.3
U. S. stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	16.2	15.5	16.4
U. S. stock/bond mutual funds | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	4.0	4.0	3.9
U. S. stock/bond mutual funds | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
Gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.2)		(1.0)
Derivative liability	0.0		0.0
Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment	0.2		1.0
Gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment	0.0		0.0
Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
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		(1.3)	(0.6)
Derivative asset		0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(13.5)	(16.4)	(14.1)
Derivative liability	0.0	0.0	0.0
Fair Value Assets Measured On Recurring Basis NYMEX Natural Gas Contracts Derivative Assets			0.0
NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment		1.3
LIABILITIES
Derivative liability before netting adjustment	13.5	16.4	14.1
Fair Value Assets Measured On Recurring Basis NYMEX Natural Gas Contracts Derivative Assets			0.6
NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment		0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Fair Value Assets Measured On Recurring Basis NYMEX Natural Gas Contracts Derivative Assets			0.0
NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before netting adjustment		0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Fair Value Assets Measured On Recurring Basis NYMEX Natural Gas Contracts Derivative Assets			0.0
OTCBB natural gas contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative liability	6.0	5.9	14.5
OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
OTCBB natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment	6.0	5.9	14.5
OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
NYMEX gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		(0.3)
Derivative liability		0.0
NYMEX gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.3
NYMEX gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.0
NYMEX gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.0
Interest rate swaps
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0
Derivative liability	1.8
Interest rate swaps | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	0.0
Interest rate swaps | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	1.8
Interest rate swaps | Significant Observable Inputs (Level 2) | Alagasco
LIABILITIES
Derivative liability before netting adjustment	2.9
Interest rate swaps | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0
Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	(1.3)	(0.6)
Derivative asset	20.3	19.5	20.3
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(13.7)	(16.7)	(15.1)
Derivative liability	6.0	5.9	17.4
Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	16.3	16.8	17.0
LIABILITIES
Derivative liability before netting adjustment	13.7	16.7	15.1
Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	4.0	3.9
LIABILITIES
Derivative liability before netting adjustment	6.0	5.9	17.4
Gas Utility | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | U. S. stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	20.2	19.5	20.3
Gas Utility | U. S. stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	16.2	15.5	16.4
Gas Utility | U. S. stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	4.0	3.9
Gas Utility | U. S. stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
Gas Utility | Gasoline and heating oil contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0
Derivative asset	0.1
Gas Utility | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.1
Gas Utility | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.0
Gas Utility | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0
Gas Utility | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(1.3)	(0.6)
Derivative asset		0.0	0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(13.5)	(16.4)	(14.1)
Derivative liability	0.0	0.0	0.0
Gas Utility | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		1.3	0.6
LIABILITIES
Derivative liability before netting adjustment	13.5	16.4	14.1
Gas Utility | NYMEX/ICE natural gas contracts | 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	0.0
Gas Utility | NYMEX/ICE natural gas contracts | 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	0.0
Gas Utility | OTCBB natural gas contracts
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative liability	6.0	5.9	14.5
Gas Utility | OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | OTCBB natural gas contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	6.0	5.9	14.5
Gas Utility | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | NYMEX gasoline and heating oil contracts
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.2)	(0.3)	(1.0)
Derivative liability	0.0	0.0	0.0
Gas Utility | NYMEX gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	0.2	0.3	1.0
Gas Utility | NYMEX gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | NYMEX gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | Interest rate swaps
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables			0.0
Derivative liability			2.9
Gas Utility | Interest rate swaps | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment			0.0
Gas Utility | Interest rate swaps | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment			2.9
Gas Utility | Interest rate swaps | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment			0.0
Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(8.4)
Derivative asset		24.7
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		(22.3)	(21.0)
Derivative liability		7.6	17.7
Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		23.1
LIABILITIES
Derivative liability before netting adjustment		17.9	17.0
Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		9.8
LIABILITIES
Derivative liability before netting adjustment		12.0	21.7
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
Gas Marketing | U. S. stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(5.9)		(5.4)
Derivative asset	31.1		23.8
Gas Marketing | U. S. stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	18.3		20.4
Gas Marketing | U. S. stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	18.5		8.3
Gas Marketing | U. S. stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.2		0.5
Gas Marketing | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(5.1)	(6.6)	(4.3)
Derivative asset	3.7	4.0	0.8
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(5.5)	(5.1)	(5.4)
Derivative liability	0.0	0.0	0.0
Gas Marketing | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	2.0	6.3	3.4
LIABILITIES
Derivative liability before netting adjustment	2.1	1.2	1.9
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	6.8	4.3	1.7
LIABILITIES
Derivative liability before netting adjustment	3.4	3.9	3.5
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Marketing | Natural gas commodity contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.8)	(0.5)	(0.5)
Derivative asset	7.1	1.2	2.7
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.8)	(0.5)	(0.5)
Derivative liability	2.9	1.7	0.3
Gas Marketing | Natural gas commodity contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Marketing | Natural gas commodity contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	7.7	1.5	2.7
LIABILITIES
Derivative liability before netting adjustment	3.7	2.2	0.8
Gas Marketing | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.2	0.2	0.5
LIABILITIES
Derivative liability before netting adjustment	$ 0.0	$ 0.0	$ 0.0