Additional loss contingency provision of customer refund.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of savings due to cost management program allowed to be retained by the entity.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Customer refund.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The revenue of off system sales attributable to customers.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum amount of annual cost savings that the utility is able to retain under cost savings plan.

+ References

No definition available.

+ Details

Name:	sr_MaximumAmountOfCostSavingsAllowable
Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Minimum number of months that is required between discretionary PGA changes.

+ References

No definition available.

+ Details

Name:	sr_MonthsBetweenPGADiscretionaryChanges
Namespace Prefix:	sr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of discretionary PGA changes the Utility can make during each fiscal year.

+ References

No definition available.

+ Details

Name:	sr_NumberofPGADiscretionaryChanges
Namespace Prefix:	sr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Public utilities annual authorized revenue.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Public utilities approved rate additional annualized revenue.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Public utilities approved rate authorized future annualized revenue.

+ References

No definition available.

+ Details

Name:	sr_PublicUtilitiesApprovedRateAuthorizedFutureAnnualizedRevenue
Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Public utilities approved rate increase decrease costs from buildings sold excluded from rate base.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Public Utilities, Approved Rate Increase (Decrease), Impact To Earnings Per Share

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Public Utilities, Approved Rate Increase (Decrease), Impact To Net Income (Loss)

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Public utilities approved rate increase decrease in regulatory assets excluded from rate base.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Public Utilities, Approved Rate Increase (Decrease), Rate Case Expenses Excluded From Rate Base

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Public utilities, approved rate increase (decrease), regulatory costs.

+ References

No definition available.

+ Details

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Namespace Prefix:	sr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Balance Type:	debit
Period Type:	instant

- Details

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Regulatory Matters - Spire Missouri - Additional Information (Details) $ / shares in Units, $ in Millions						1 Months Ended	6 Months Ended	12 Months Ended
	Nov. 12, 2020 USD ($)	Nov. 16, 2019 USD ($)	May 25, 2019 USD ($)	Apr. 25, 2018 USD ($)	Mar. 07, 2018 USD ($) $ / shares	Aug. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) operating-segment	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Apr. 18, 2018
Public Utilities, General Disclosures [Line Items]
Customer Share								75.00%
Regulatory costs								$ 0.0	$ 0.0	$ 30.6
Provision for ISRS rulings								0.0	12.2	0.0
Regulatory assets								$ 69.5	78.6
Minimum
Public Utilities, General Disclosures [Line Items]
Customer Share											70.00%
Maximum
Public Utilities, General Disclosures [Line Items]
Customer Share											85.00%
MoPSC \| Infrastructure System Replacement Surcharge
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease), costs from buildings sold in 2014 excluded from rate base				$ (1.8)
Rate case expenses				(0.9)
Regulatory costs										$ 38.4
After-tax rate reduction to net income					$ 23.6
After-tax rate reduction to earnings per share \| $ / shares					$ 0.49
MoPSC \| Infrastructure System Replacement Surcharge \| Pension Cost
Public Utilities, General Disclosures [Line Items]
Regulatory assets recovery				$ (28.8)
Regulatory costs					$ 30.6
Spire Missouri
Public Utilities, General Disclosures [Line Items]
Number of PGA Discretionary Changes \| operating-segment								3
Months between PGA Discretionary Changes								2 months
Customer Share								25.00%
Regulatory assets								$ 32.1	29.4
Spire Missouri \| MoPSC
Public Utilities, General Disclosures [Line Items]
Cost savings retention percentage								10.00%
Maximum amount of cost savings allowable								$ 3.0
Spire Missouri \| MoPSC \| Infrastructure System Replacement Surcharge
Public Utilities, General Disclosures [Line Items]
Approved rate authorized future annualized revenue								40.3
Payment of customer refund						$ 15.0
Provision for ISRS rulings									$ 12.2
Additional loss contingency provision of customer refund							$ 4.8
Annual authorized revenue		$ 8.8	$ 12.4
Spire Missouri \| MoPSC \| Infrastructure System Replacement Surcharge \| Subsequent Event
Public Utilities, General Disclosures [Line Items]
Approved rate additional annualized revenue	$ 7.0
Spire Missouri \| MoPSC \| Accounting Authority Order
Public Utilities, General Disclosures [Line Items]
Regulatory assets								$ 3.8