Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
ASSETS
Effects of netting and cash margin receivables/payables	$ (18.8)	$ (8.4)	$ (10.9)
Derivative asset after the effects of netting	56.1	48.2	27.2
LIABILITIES
Effects of netting and cash margin receivables/payables	(18.6)	(14.8)	(14.0)
Derivative liability after the effects of netting	24.6	6.2	5.8
Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(1.2)	(2.7)	(0.5)
Derivative asset after the effects of netting	18.3	20.3	23.5
US stock/bond mutual funds \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	18.3	20.3	23.5
NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(1.2)	(2.7)	(0.5)
Derivative asset after the effects of netting	0.0	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.9)	(1.9)	(2.7)
Derivative liability after the effects of netting	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	32.2	32.1	22.4
LIABILITIES
Derivative liability before effects of netting	1.5	2.8	3.8
Quoted Prices in Active Markets (Level 1) \| Spire Missouri
ASSETS
Derivative asset before effects of netting	19.5	23.0	20.0
Quoted Prices in Active Markets (Level 1) \| US stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	18.3	20.3	19.5
Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting	1.2	2.7	0.5
LIABILITIES
Derivative liability before effects of netting	0.9	1.9	2.7
Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	42.7	24.5	15.7
LIABILITIES
Derivative liability before effects of netting	41.4	18.0	15.6
Significant Observable Inputs (Level 2) \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	4.0
Significant Observable Inputs (Level 2) \| US stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	4.0
Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.3	0.2	0.4
Significant Unobservable Inputs (Level 3) \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3) \| US stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Corporate, Non-Segment \| 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	12.5	8.9
Corporate, Non-Segment \| Interest rate swaps
ASSETS
Effects of netting and cash margin receivables/payables		0.0
Derivative asset after the effects of netting		3.0
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0		0.0
Derivative liability after the effects of netting	7.5		0.8
Corporate, Non-Segment \| Quoted Prices in Active Markets (Level 1) \| US stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	12.5	8.9
Corporate, Non-Segment \| Quoted Prices in Active Markets (Level 1) \| Interest rate swaps
ASSETS
Derivative asset before effects of netting		0.0
LIABILITIES
Derivative liability before effects of netting	0.0		0.0
Corporate, Non-Segment \| Significant Observable Inputs (Level 2) \| US stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Corporate, Non-Segment \| Significant Observable Inputs (Level 2) \| Interest rate swaps
ASSETS
Derivative asset before effects of netting		3.0
LIABILITIES
Derivative liability before effects of netting	7.5		0.8
Corporate, Non-Segment \| Significant Unobservable Inputs (Level 3) \| US stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Corporate, Non-Segment \| Significant Unobservable Inputs (Level 3) \| Interest rate swaps
ASSETS
Derivative asset before effects of netting		0.0
LIABILITIES
Derivative liability before effects of netting	0.0		0.0
Gas Utility \| Operating Segments \| US stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	18.3	20.3	23.5
Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(1.2)	(2.7)	(0.5)
Derivative asset after the effects of netting	0.0	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.9)	(1.9)	(2.7)
Derivative liability after the effects of netting	0.0	0.0	0.0
Gas Utility \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| US stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	18.3	20.3	19.5
Gas Utility \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	1.2	2.7	0.5
LIABILITIES
Derivative liability before effects of netting	0.9	1.9	2.7
Gas Utility \| Operating Segments \| Significant Observable Inputs (Level 2) \| US stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	4.0
Gas Utility \| Operating Segments \| Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Utility \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| US stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Gas Utility \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(9.3)	(4.2)	(6.1)
Derivative asset after the effects of netting	2.8	0.0	0.3
LIABILITIES
Effects of netting and cash margin receivables/payables	(9.4)	(11.4)	(7.0)
Derivative liability after the effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(8.3)	(1.5)	(4.3)
Derivative asset after the effects of netting	22.5	16.0	3.4
LIABILITIES
Effects of netting and cash margin receivables/payables	(8.3)	(1.5)	(4.3)
Derivative liability after the effects of netting	17.1	6.2	5.0
Gas Marketing \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.2	0.2	2.4
LIABILITIES
Derivative liability before effects of netting	0.6	0.9	1.1
Gas Marketing \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	11.9	4.0	4.0
LIABILITIES
Derivative liability before effects of netting	8.8	10.5	5.9
Gas Marketing \| Operating Segments \| Significant Observable Inputs (Level 2) \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	30.8	17.5	7.7
LIABILITIES
Derivative liability before effects of netting	25.1	7.5	8.9
Gas Marketing \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	$ 0.3	$ 0.2	$ 0.4

No definition available.

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

+ References

+ Details

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

- Definition

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssetFairValueGrossLiabilityAndObligationToReturnCashOffset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and right to receive cash collateral under master netting arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilityFairValueGrossLiabilityIncludingNotSubjectToMasterNettingArrangement
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
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_StatementBusinessSegmentsAxis=sr_GasUtilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=sr_GasMarketingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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