FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
ASSETS
Derivative asset after the effects of netting	$ 5.2	$ 2.8
Laclede Gas
ASSETS
Derivative asset after the effects of netting	0.0	(0.4)
Fair Value, Measurements, Recurring | Laclede Gas
ASSETS
Effects of netting and cash margin receivables/payables	(1.3)	(2.5)
Derivative asset after the effects of netting	19.5	19.6
LIABILITIES
Effects of netting and cash margin receivables/payables	(16.7)	(5.5)
Derivative liability after the effects of netting	5.9	4.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before effects of netting	16.8	18.1
LIABILITIES
Derivative liability before effects of netting	16.7	5.4
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before effects of netting	4.0	4.0
LIABILITIES
Derivative liability before effects of netting	5.9	4.1
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | U. S. Stock/Bond Mutual Funds | Laclede Gas
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	19.5	19.6
Fair Value, Measurements, Recurring | U. S. Stock/Bond Mutual Funds | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before effects of netting	15.5	15.7
Fair Value, Measurements, Recurring | U. S. Stock/Bond Mutual Funds | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before effects of netting	4.0	3.9
Fair Value, Measurements, Recurring | U. S. Stock/Bond Mutual Funds | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Laclede Gas
ASSETS
Effects of netting and cash margin receivables/payables	(1.3)	(2.4)
Derivative asset after the effects of netting	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(16.4)	(5.2)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before effects of netting	1.3	2.4
LIABILITIES
Derivative liability before effects of netting	16.4	5.2
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Laclede Gas
ASSETS
Effects of netting and cash margin receivables/payables		(0.1)
Derivative asset after the effects of netting		0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	(0.1)
Derivative liability after the effects of netting	5.9	4.0
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
ASSETS
Derivative asset before effects of netting		0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Significant Observable Inputs (Level 2) | Laclede Gas
ASSETS
Derivative asset before effects of netting		0.1
LIABILITIES
Derivative liability before effects of netting	5.9	4.1
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
ASSETS
Derivative asset before effects of netting		0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.3)
Derivative liability after the effects of netting	0.0
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
LIABILITIES
Derivative liability before effects of netting	0.3
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Laclede Gas
LIABILITIES
Derivative liability before effects of netting	0.0
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
LIABILITIES
Derivative liability before effects of netting	0.0
Fair Value, Measurements, Recurring | NYMEX gasoline and heating oil contracts | Laclede Gas
LIABILITIES
Effects of netting and cash margin receivables/payables		(0.2)
Derivative liability after the effects of netting		0.0
Fair Value, Measurements, Recurring | NYMEX gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Laclede Gas
LIABILITIES
Derivative liability before effects of netting		0.2
Fair Value, Measurements, Recurring | NYMEX gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Laclede Gas
LIABILITIES
Derivative liability before effects of netting		0.0
Fair Value, Measurements, Recurring | NYMEX gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Laclede Gas
LIABILITIES
Derivative liability before effects of netting		0.0
Fair Value, Measurements, Recurring | Gas Utility
ASSETS
Effects of netting and cash margin receivables/payables	(1.3)	(2.5)
Derivative asset after the effects of netting	19.5	19.6
LIABILITIES
Effects of netting and cash margin receivables/payables	(16.7)	(5.5)
Derivative liability after the effects of netting	5.9	4.0
Fair Value, Measurements, Recurring | Gas Utility | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	16.8	18.1
LIABILITIES
Derivative liability before effects of netting	16.7	5.4
Fair Value, Measurements, Recurring | Gas Utility | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	4.0	4.0
LIABILITIES
Derivative liability before effects of netting	5.9	4.1
Fair Value, Measurements, Recurring | Gas Utility | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | U. S. Stock/Bond Mutual Funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	19.5	19.6
Fair Value, Measurements, Recurring | Gas Utility | U. S. Stock/Bond Mutual Funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	15.5	15.7
Fair Value, Measurements, Recurring | Gas Utility | U. S. Stock/Bond Mutual Funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	4.0	3.9
Fair Value, Measurements, Recurring | Gas Utility | U. S. Stock/Bond Mutual Funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(1.3)	(2.4)
Derivative asset after the effects of netting	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(16.4)	(5.2)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	1.3	2.4
LIABILITIES
Derivative liability before effects of netting	16.4	5.2
Fair Value, Measurements, Recurring | Gas Utility | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | OTCBB natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables		(0.1)
Derivative asset after the effects of netting		0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	(0.1)
Derivative liability after the effects of netting	5.9	4.0
Fair Value, Measurements, Recurring | Gas Utility | OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting		0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | OTCBB natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting		0.1
LIABILITIES
Derivative liability before effects of netting	5.9	4.1
Fair Value, Measurements, Recurring | Gas Utility | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting		0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | NYMEX gasoline and heating oil contracts
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.3)	(0.2)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | NYMEX gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before effects of netting	0.3	0.2
Fair Value, Measurements, Recurring | Gas Utility | NYMEX gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility | NYMEX gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing
ASSETS
Effects of netting and cash margin receivables/payables	(8.4)	(4.5)
Derivative asset after the effects of netting	24.7	22.7
LIABILITIES
Effects of netting and cash margin receivables/payables	(22.3)	(7.5)
Derivative liability after the effects of netting	7.6	4.5
Fair Value, Measurements, Recurring | Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	23.1	19.1
LIABILITIES
Derivative liability before effects of netting	17.9	6.5
Fair Value, Measurements, Recurring | Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	9.8	7.9
LIABILITIES
Derivative liability before effects of netting	12.0	5.5
Fair Value, Measurements, Recurring | Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.2	0.2
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(6.6)
Derivative asset after the effects of netting	4.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(5.1)	(1.8)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	6.3
LIABILITIES
Derivative liability before effects of netting	1.2	1.1
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	4.3
LIABILITIES
Derivative liability before effects of netting	3.9	0.7
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables		(1.8)
Derivative asset after the effects of netting		0.4
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting		1.0
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting		1.2
Fair Value, Measurements, Recurring | Gas Marketing | NYMEX natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | Gas Marketing | Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(0.5)	(0.2)
Derivative asset after the effects of netting	1.2	2.7
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.5)	(0.2)
Derivative liability after the effects of netting	1.7	0.5
Fair Value, Measurements, Recurring | Gas Marketing | Natural gas commodity contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing | Natural gas commodity contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	1.5	2.7
LIABILITIES
Derivative liability before effects of netting	2.2	0.7
Fair Value, Measurements, Recurring | Gas Marketing | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.2	0.2
LIABILITIES
Derivative liability before effects of netting	$ 0.0	$ 0.0