FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
ASSETS
Derivative asset after the effects of netting	$ 6.2	$ 13.3
LIABILITIES
Derivative liability after the effects of netting	8.0	4.9
Spire Missouri
ASSETS
Derivative asset after the effects of netting	0.1	4.9
LIABILITIES
Derivative liability after the effects of netting	0.0	0.2
Fair Value, Measurements, Recurring
ASSETS
Effects of netting and cash margin receivables/payables	(6.7)	(5.0)
Derivative asset after the effects of netting	28.7	34.3
LIABILITIES
Effects of netting and cash margin receivables/payables	(5.2)
Derivative liability after the effects of netting	8.1
Fair Value, Measurements, Recurring | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(3.4)	(0.7)
Derivative asset after the effects of netting	22.5	25.8
LIABILITIES
Effects of netting and cash margin receivables/payables	(1.9)	(1.6)
Derivative liability after the effects of netting	0.0	0.2
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	23.1	22.9
LIABILITIES
Derivative liability before effects of netting	3.7
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting	21.8	22.4
LIABILITIES
Derivative liability before effects of netting	1.9	1.6
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	12.2	16.2
LIABILITIES
Derivative liability before effects of netting	9.6
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting	4.1	4.1
LIABILITIES
Derivative liability before effects of netting	0.0	0.2
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.1	0.2
LIABILITIES
Derivative liability before effects of netting	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Spire Missouri
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 | US stock/bond mutual funds | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	22.4	20.9
Fair Value, Measurements, Recurring | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting	18.3	16.8
Fair Value, Measurements, Recurring | US stock/bond mutual funds | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting	4.1	4.1
Fair Value, Measurements, Recurring | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(3.4)	(0.4)
Derivative asset after the effects of netting	0.0	4.9
LIABILITIES
Effects of netting and cash margin receivables/payables	(1.9)	(1.6)
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) | Spire Missouri
ASSETS
Derivative asset before effects of netting	3.4	5.3
LIABILITIES
Derivative liability before effects of netting	1.9	1.6
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2) | Spire Missouri
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) | Spire Missouri
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 | Spire Missouri
LIABILITIES
Effects of netting and cash margin receivables/payables		0.0
Derivative liability after the effects of netting		0.2
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1) | Spire Missouri
LIABILITIES
Derivative liability before effects of netting		0.0
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Significant Observable Inputs (Level 2) | Spire Missouri
LIABILITIES
Derivative liability before effects of netting		0.2
Fair Value, Measurements, Recurring | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3) | Spire Missouri
LIABILITIES
Derivative liability before effects of netting		0.0
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0.0	(0.3)
Derivative asset after the effects of netting	0.1	0.0
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.1	0.3
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts
ASSETS
Effects of netting and cash margin receivables/payables		(0.3)
Derivative asset after the effects of netting		0.1
Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting		0.4
Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | Gas Marketing:
LIABILITIES
Effects of netting and cash margin receivables/payables		7.6
Derivative liability after the effects of netting		4.9
Fair Value, Measurements, Recurring | Gas Marketing: | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before effects of netting		5.1
Fair Value, Measurements, Recurring | Gas Marketing: | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before effects of netting		7.4
Fair Value, Measurements, Recurring | Gas Marketing: | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before effects of netting		0.0
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | US stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	22.4	20.9
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	18.3	16.8
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	4.1	4.1
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(3.4)	(0.4)
Derivative asset after the effects of netting	0.0	4.9
LIABILITIES
Effects of netting and cash margin receivables/payables	(1.9)	(1.6)
Derivative liability after the effects of netting	0.0	0.0
Operating Segments | 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	3.4	5.3
LIABILITIES
Derivative liability before effects of netting	1.9	1.6
Operating Segments | 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
Operating Segments | 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
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | OTCBB natural gas contracts
LIABILITIES
Effects of netting and cash margin receivables/payables		0.0
Derivative liability after the effects of netting		0.2
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | OTCBB natural gas contracts | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before effects of netting		0.0
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | OTCBB natural gas contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before effects of netting		0.2
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | OTCBB natural gas contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before effects of netting		0.0
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts
ASSETS
Effects of netting and cash margin receivables/payables	0.0
Derivative asset after the effects of netting	0.1
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	0.1
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	0.0
Operating Segments | Fair Value, Measurements, Recurring | Gas Utility: | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(2.1)
Derivative asset after the effects of netting	0.5
LIABILITIES
Effects of netting and cash margin receivables/payables	(2.1)	(5.1)
Derivative liability after the effects of netting	0.0	0.0
Operating Segments | 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	1.3
LIABILITIES
Derivative liability before effects of netting	1.8	3.5
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	1.3
LIABILITIES
Derivative liability before effects of netting	0.3	1.6
Operating Segments | 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
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables		(3.4)
Derivative asset after the effects of netting		0.4
Operating Segments | 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		0.4
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting		3.4
Operating Segments | 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
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(1.2)	(0.9)
Derivative asset after the effects of netting	5.7	8.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(1.2)	(0.9)
Derivative liability after the effects of netting	7.2	1.7
Operating Segments | 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
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | Natural gas commodity contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	6.8	8.7
LIABILITIES
Derivative liability before effects of netting	8.4	2.6
Operating Segments | Fair Value, Measurements, Recurring | Gas Marketing: | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.1	0.2
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Other | Fair Value, Measurements, Recurring | Interest rate swaps
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative liability after the effects of netting	0.9	3.0
Other | Fair Value, Measurements, Recurring | Interest rate swaps | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Other | Fair Value, Measurements, Recurring | Interest rate swaps | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before effects of netting	0.9	3.0
Other | Fair Value, Measurements, Recurring | Interest rate swaps | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before effects of netting	$ 0.0	$ 0.0