FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	$ (5.6)	$ (7.6)
Derivative liability	4.4	4.9
Laclede Gas
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.3)	(0.7)	$ (3.1)
Derivative asset	22.8	25.8	25.5
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(1.3)	(1.6)	0.0
Derivative liability	0.0	0.2	0.6
Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	2.1	5.1
Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	20.0	22.4	24.6
LIABILITIES
Derivative liability before netting adjustment	1.3	1.6	0.0
Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	7.9	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	0.6
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	22.1	20.9	20.4
US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	18.0	16.8	16.4
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	(1.3)	(0.4)	(3.1)
Derivative asset	0.6	4.9	5.1
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(1.3)	(1.6)
Derivative liability	0.0	0.0
NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	1.9	5.3	8.2
LIABILITIES
Derivative liability before netting adjustment	1.3	1.6
NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	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	0.0
LIABILITIES
Derivative liability before netting adjustment	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	0.1	0.0
Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.1	0.3
Gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	0.0
Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before netting adjustment	0.0	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	0.6
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	0.6
OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Interest rate swaps
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0
Derivative liability	(0.6)
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	(0.6)
Interest rate swaps | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0
Other | Interest rate swaps
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative liability		3.0	4.0
Other | Interest rate swaps | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Other | Interest rate swaps | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment		3.0	4.0
Other | Interest rate swaps | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Utility
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.3)	(0.7)	(3.1)
Derivative asset	22.8	25.9	25.5
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		(1.6)	0.0
Derivative liability		0.2	0.6
Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	20.0	22.5	24.6
LIABILITIES
Derivative liability before netting adjustment		1.6	0.0
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		0.2	0.6
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
Gas Utility | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	22.1	20.9	20.4
Gas Utility | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	18.0	16.8	16.4
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	(1.3)	(0.4)	(3.1)
Derivative asset	0.6	4.9	5.1
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(1.3)	(1.6)
Derivative liability	0.0	0.0
Gas Utility | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	1.9	5.3	8.2
LIABILITIES
Derivative liability before netting adjustment	1.3	1.6
Gas Utility | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	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	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Gas Utility | Gasoline and heating oil contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	(0.3)
Derivative asset	0.1	0.1
Gas Utility | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.1	0.4
Gas Utility | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	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
Gas Utility | OTCBB natural gas contracts
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative liability		0.2	0.6
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	0.6
Gas Utility | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(5.6)
Derivative asset	29.9
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables			(9.5)
Derivative liability			7.5
Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	21.5
LIABILITIES
Derivative liability before netting adjustment			6.5
Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	13.4
LIABILITIES
Derivative liability before netting adjustment			10.5
Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.6
LIABILITIES
Derivative liability before netting adjustment			0.0
Gas Marketing | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(5.0)	(9.3)
Derivative asset		34.3	32.0
Gas Marketing | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		22.9	25.2
Gas Marketing | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		16.2	15.8
Gas Marketing | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.2	0.3
Gas Marketing | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(2.6)	(3.4)	(5.6)
Derivative asset	2.1	0.4	0.5
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(2.6)	(5.1)	(9.0)
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	1.5	0.4	0.6
LIABILITIES
Derivative liability before netting adjustment	0.8	3.5	6.5
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	3.2	3.4	5.5
LIABILITIES
Derivative liability before netting adjustment	1.8	1.6	2.5
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	(1.7)	(0.9)	(0.6)
Derivative asset	5.0	8.0	6.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(0.9)	(0.5)
Derivative liability	5.0	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	6.1	8.7	6.3
LIABILITIES
Derivative liability before netting adjustment	6.7	2.6	3.4
Gas Marketing | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.6	0.2	0.3
LIABILITIES
Derivative liability before netting adjustment	$ 0.0	$ 0.0	$ 0.0