DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES Narrative (Details) gal in Millions, MMBTU in Millions		12 Months Ended
	May 21, 2015 USD ($)	Sep. 30, 2017 USD ($) MMBTU $ / gal gal	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 01, 2015 USD ($)	Sep. 15, 2015 USD ($)	Apr. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)
Derivative [Line Items]
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) | MMBTU		202.1
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2018 | MMBTU		156.8
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2019 | MMBTU		34.3
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2020 | MMBTU		5.9
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2021 | MMBTU		4.1
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2022 | MMBTU		0.9
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2023 | MMBTU		0.1
Unrealized gain on energy-related derivatives		$ (6,000,000)	$ 100,000	$ 2,800,000
Gain (loss) to be reclassified within twelve months		0
Cash margin receivables not offset with derivatives		$ 4,000,000	2,900,000
Spire Alabama
Derivative [Line Items]
Unsecured debt				115,000,000				$ 115,000,000
Gasoline futures contracts notional amount (in gallons) | gal		0.1
Average price of future contracts (dollars per gallon) | $ / gal		1.28
Spire Alabama | 3.21% Notes, due September 15, 2025
Derivative [Line Items]
Unsecured debt						$ 35,000,000
Spire Alabama | Notes 4.31%, Due December 1, 2045
Derivative [Line Items]
Unsecured debt					$ 80,000,000
Spire Missouri
Derivative [Line Items]
MMBtu (millions) | MMBTU		33.9
Gain (loss) to be reclassified within twelve months		$ 0
Cash margin receivables not offset with derivatives		$ 4,000,000	500,000
Gasoline futures contracts notional amount (in gallons) | gal		0.3
Average price of future contracts (dollars per gallon) | $ / gal		1.27
Designated as Hedging Instrument | Interest rate swaps | Spire Alabama
Derivative [Line Items]
Loss on termination and settlement of interest rate swap		$ 900,000
Gain on settlement of derivative	$ 2,700,000	14,100,000		2,700,000
Designated as Hedging Instrument | Interest rate swaps | Spire Energy South Inc
Derivative [Line Items]
Loss on termination and settlement of interest rate swap			400,000
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps
Derivative [Line Items]
Notional amount							$ 24,000,000.0
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Alabama
Derivative [Line Items]
Notional amount		$ 60,000,000	225,000,000	$ 104,500,000				$ 80,500,000
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Alabama | Minimum
Derivative [Line Items]
Derivative fixed interest rate		265.80%						2.18%
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Alabama | Maximum
Derivative [Line Items]
Derivative fixed interest rate								2.85%
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Energy South Inc
Derivative [Line Items]
Notional amount			$ 85,000,000