FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	$ (7.0)	$ (7.6)
Derivative liability	6.0	4.9
Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.8)	(0.7)	$ 0.0
Derivative asset	24.7	25.8	20.2
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(1.6)	(13.7)
Derivative liability	0.0	0.2	6.0
Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	2.6	5.1
Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	22.4	22.4	16.2
LIABILITIES
Derivative liability before netting adjustment	0.5	1.6	13.7
Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	10.4	7.4
Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.2	6.0
Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0	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
US stock/bond mutual funds | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	21.8	20.9	20.2
US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	17.7	16.8	16.2
US stock/bond mutual funds | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.0
US 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	(1.8)	(0.4)
Derivative asset	2.7	4.9
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(1.6)	(13.5)
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	4.5	5.3
LIABILITIES
Derivative liability before netting adjustment	0.5	1.6	13.5
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
Gasoline and heating oil contracts | Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	(0.3)
Derivative asset	0.2	0.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables			(0.2)
Derivative liability			0.0
Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.2	0.3
LIABILITIES
Derivative liability before netting adjustment			0.2
Gasoline and heating oil 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
Gasoline and heating oil 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
OTCBB natural gas contracts | Laclede Gas
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative liability		0.2	6.0
OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
OTCBB natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.2	6.0
OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Other | Interest rate swaps
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0
Derivative asset	0.2
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative liability		3.0	1.8
Other | Interest rate swaps | 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
Other | Interest rate swaps | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.2
LIABILITIES
Derivative liability before netting adjustment		3.0	1.8
Other | Interest rate swaps | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.8)	(0.7)	0.0
Derivative asset	24.7	25.9	20.3
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(1.6)	(13.7)
Derivative liability	0.0	0.2	6.0
Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	22.4	22.5	16.3
LIABILITIES
Derivative liability before netting adjustment	0.5	1.6	13.7
Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.2	6.0
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 | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	21.8	20.9	20.2
Gas Utility | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	17.7	16.8	16.2
Gas Utility | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.0
Gas Utility | US 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	(1.8)	(0.4)
Derivative asset	2.7	4.9
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(1.6)	(13.5)
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	4.5	5.3
LIABILITIES
Derivative liability before netting adjustment	0.5	1.6	13.5
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 | Gasoline and heating oil contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	(0.3)	0.0
Derivative asset	0.2	0.1	0.1
Gas Utility | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.2	0.4	0.1
Gas Utility | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
Gas Utility | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset 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
Derivative liability		0.2	6.0
Gas Utility | OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Utility | OTCBB natural gas contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment		0.2	6.0
Gas Utility | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Utility | NYMEX gasoline and heating oil contracts
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables			(0.2)
Derivative liability			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
Gas Utility | NYMEX gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment			0.0
Gas Utility | NYMEX gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment			0.0
Gas Marketing
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables			(20.0)
Derivative liability			10.7
Gas Marketing | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment			15.8
Gas Marketing | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment			14.9
Gas Marketing | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment			0.0
Gas Marketing | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(6.7)	(5.0)	(5.9)
Derivative asset	31.0	34.3	31.1
Gas Marketing | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	23.4	22.9	18.3
Gas Marketing | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	14.1	16.2	18.5
Gas Marketing | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.2	0.2	0.2
Gas Marketing | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(4.4)	(3.4)	(5.1)
Derivative asset	0.2	0.4	3.7
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(6.0)	(5.1)	(5.5)
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	1.0	0.4	2.0
LIABILITIES
Derivative liability before netting adjustment	2.1	3.5	2.1
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	3.6	3.4	6.8
LIABILITIES
Derivative liability before netting adjustment	3.9	1.6	3.4
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	(0.5)	(0.9)	(0.8)
Derivative asset	5.9	8.0	7.1
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(0.9)	(0.8)
Derivative liability	6.0	1.7	2.9
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	6.2	8.7	7.7
LIABILITIES
Derivative liability before netting adjustment	6.5	2.6	3.7
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