Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate notional amount of natural gas derivative contracts not designated as normal purchases or normal sales with notional amounts expressed in nonmonetary units. For example the number of MMBtus specified in a natural gas forward purchase contract.

+ References

No definition available.

+ Details

Name:	lg_NonmonetaryNotionalAmountOfNaturalGasDerivativeContractsNotDesignatedAsNormalPurchasesOrNormalSales
Namespace Prefix:	lg_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in The Next Twelve Months

+ References

No definition available.

+ Details

Name:	lg_NotionalAmountofNonExchangeTradedNaturalGasCommodityContractsToBeSettledinTheNextTwelveMonths
Namespace Prefix:	lg_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in Year Four

+ References

No definition available.

+ Details

Name:	lg_NotionalAmountofNonExchangeTradedNaturalGasCommodityContractsToBeSettledinYearFour
Namespace Prefix:	lg_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in Year Six

+ References

No definition available.

+ Details

Name:	lg_NotionalAmountofNonExchangeTradedNaturalGasCommodityContractsToBeSettledinYearSix
Namespace Prefix:	lg_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in Year Three

+ References

No definition available.

+ Details

Name:	lg_NotionalAmountofNonExchangeTradedNaturalGasCommodityContractsToBeSettledinYearThree
Namespace Prefix:	lg_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in Year Two

+ References

No definition available.

+ Details

Name:	lg_NotionalAmountofNonExchangeTradedNaturalGasCommodityContractsToBeSettledinYearTwo
Namespace Prefix:	lg_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

+ Details

Name:	us-gaap_DerivativeGainOnDerivative
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of decrease in the fair value of derivatives recognized in the income statement.

+ References

+ Details

Name:	us-gaap_DerivativeLossOnDerivative
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
Namespace Prefix:	us-gaap_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of other receivables due from broker-dealers or clearing organizations.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=lg_SpireMissouriMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=lg_SpireEnergySouthIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Narrative (Details) MMBTU in Millions, $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) MMBTU	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Derivative [Line Items]
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) \| MMBTU		277.7
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2018 (in MMBtu) \| MMBTU		196.7
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2019 (in MMBtu) \| MMBTU		43.1
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2020 (in MMBtu) \| MMBTU		30.5
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2021 (in MMBtu) \| MMBTU		7.3
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2023 (in MMBtu) \| MMBTU		0.1
Mark-to-market gain		$ 4.0	$ (6.0)	$ 0.1
Cash margin receivables not offset with derivatives		$ 4.1	4.0
Spire Missouri
Derivative [Line Items]
MMBtu (millions) \| MMBTU		38.4
Cash margin receivables not offset with derivatives			3.8	4.0
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 100.0
Derivative fixed interest rate	2.7675%
Derivative, term of contract	3 years
Mark-to-market gain		$ 0.5
Derivatives in Cash Flow Hedging Relationships \| Interest rate swaps
Derivative [Line Items]
Loss on termination and settlement of interest rate swap		$ 2.5
Gain on settlement of derivative			14.1
Derivatives in Cash Flow Hedging Relationships \| Interest rate swaps \| Spire EnergySouth Inc
Derivative [Line Items]
Loss on termination and settlement of interest rate swap				0.4
Cash Flow Hedging \| Derivatives in Cash Flow Hedging Relationships \| Interest rate swaps
Derivative [Line Items]
Notional amount			$ 60.0	225.0
Derivative fixed interest rate			2.658%
Cash Flow Hedging \| Derivatives in Cash Flow Hedging Relationships \| Interest rate swaps \| Spire EnergySouth Inc
Derivative [Line Items]
Notional amount				$ 85.0