FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	$ (0.1)	$ (1.3)	$ 0.0
Derivative asset	19.9	19.5	20.2
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(17.4)	(16.7)	(20.7)
Derivative liability	4.6	5.9	19.5
Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	16.0	16.8	16.3
LIABILITIES
Derivative liability before netting adjustment	17.4	16.7	20.7
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	4.6	5.9	19.5
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	19.9	19.5	20.2
U. S. stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	15.9	15.5	16.3
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
NYMEX/ICE natural gas contracts | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.1)	(1.3)
Derivative asset	0.0	0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(17.3)	(16.4)	(19.4)
Derivative liability	0.0	0.0	0.0
NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.1	1.3
LIABILITIES
Derivative liability before netting adjustment	17.3	16.4	19.4
NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	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	4.6	5.9	19.5
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	4.6	5.9	19.5
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
Gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.1)		(1.3)
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	0.1		1.3
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
Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.1)	(1.3)
Derivative asset	19.9	19.5
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(17.4)	(16.7)	(20.7)
Derivative liability	4.6	5.9	19.5
Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	16.0	16.8
LIABILITIES
Derivative liability before netting adjustment	17.4	16.7	20.7
Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	4.0
LIABILITIES
Derivative liability before netting adjustment	4.6	5.9	19.5
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	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	19.9	19.5	20.2
Gas Utility | U. S. stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	15.9	15.5	16.3
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 | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.1)	(1.3)
Derivative asset	0.0	0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(17.3)	(16.4)	(19.4)
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	0.1	1.3
LIABILITIES
Derivative liability before netting adjustment	17.3	16.4	19.4
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	4.6	5.9	19.5
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	4.6	5.9	19.5
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	(0.1)	(0.3)	(1.3)
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	0.1	0.3	1.3
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 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	(22.7)	(22.3)	(28.9)
Derivative liability	5.7	7.6	20.3
Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		23.1
LIABILITIES
Derivative liability before netting adjustment	18.9	17.9	24.1
Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		9.8
LIABILITIES
Derivative liability before netting adjustment	9.5	12.0	25.1
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	0.0
Gas Marketing | U. S. stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(7.3)		(4.6)
Derivative asset	24.7		26.3
Gas Marketing | U. S. stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	20.1		21.7
Gas Marketing | U. S. stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	11.7		9.0
Gas Marketing | U. S. stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.2		0.2
Gas Marketing | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(6.9)	(6.6)	(4.3)
Derivative asset	3.0	4.0	2.1
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(5.0)	(5.1)	(7.9)
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	4.1	6.3	5.4
LIABILITIES
Derivative liability before netting adjustment	1.5	1.2	3.4
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	5.8	4.3	1.0
LIABILITIES
Derivative liability before netting adjustment	3.5	3.9	4.5
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	(0.3)	(0.5)	(0.3)
Derivative asset	1.8	1.2	4.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.3)	(0.5)	(0.3)
Derivative liability	1.1	1.7	0.8
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	1.9	1.5	4.1
LIABILITIES
Derivative liability before netting adjustment	1.4	2.2	1.1
Gas Marketing | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.2	0.2	0.2
LIABILITIES
Derivative liability before netting adjustment	$ 0.0	$ 0.0	$ 0.0