DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	$ 13.0	$ 18.4
Fair value of liability derivatives presented above	13.3	12.5
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.7
Fair value of liability derivatives presented above	1.1
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	12.3	15.2
Fair value of liability derivatives presented above	12.2	8.6
Gas Utility: | Operating Segments | Gasoline and heating oil contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.1	0.3
Fair value of liability derivatives presented above	0.0	0.0
Gas Utility: | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		0.2
Gas Utility: | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.4	5.4
Fair value of liability derivatives presented above	1.9	1.6
Gas Marketing: | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		3.2
Fair value of liability derivatives presented above		3.9
Gas Marketing: | Natural gas contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		2.5
Fair value of liability derivatives presented above		0.8
Gas Marketing: | Interest rate swaps | Designated as Hedging Instrument | Current Liabilities – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.0
Fair value of liability derivatives presented above	0.9
Gas Marketing: | Operating Segments | Natural gas contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.3
Fair value of liability derivatives presented above	0.2
Gas Marketing: | Operating Segments | Natural gas contracts | Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.3	0.4
Fair value of liability derivatives presented above	0.0	0.1
Gas Marketing: | Operating Segments | Interest rate swaps | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		3.0
Gas Marketing: | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	1.7	0.8
Fair value of liability derivatives presented above	1.4	4.1
Gas Marketing: | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.3	0.0
Fair value of liability derivatives presented above	0.5	0.1
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	5.3	6.5
Fair value of liability derivatives presented above	0.1	0.2
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.4	2.1
Fair value of liability derivatives presented above	0.0	0.3
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Current Liabilities – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.8	0.2
Fair value of liability derivatives presented above	5.0	2.0
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Credits – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.4	0.2
Fair value of liability derivatives presented above	3.3	0.1
Spire Missouri
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.5	5.7
Fair value of liability derivatives presented above	1.9	1.8
Spire Missouri | Gas Utility:
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.5	5.7
Fair value of liability derivatives presented above	1.9	1.8
Spire Missouri | Gas Utility: | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.3
Fair value of liability derivatives presented above		0.0
Spire Missouri | Gas Utility: | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		5.4
Fair value of liability derivatives presented above		1.8
Spire Missouri | Gas Utility: | Gasoline and heating oil contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.1
Fair value of liability derivatives presented above	0.0
Spire Missouri | Gas Utility: | Gasoline and heating oil contracts | Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.3
Fair value of liability derivatives presented above		0.0
Spire Missouri | Gas Utility: | Natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		0.2
Spire Missouri | Gas Utility: | Natural gas contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.4	5.4
Fair value of liability derivatives presented above	$ 1.9	$ 1.6