Financing -Schedule of Short-term Borrowings (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Spire Alabama \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 59.6
Highest borrowings outstanding	$ 152.2
Weighted average interest rate	0.20%
Borrowings outstanding		$ 121.3	$ 33.0
Weighted average interest rate		0.20%	1.90%
Spire Alabama \| Revolving Credit Facility Borrowings
Short-term Debt [Line Items]
Borrowings outstanding			$ 50.0
Weighted average interest rate			1.90%
Spire Missouri \| Term Loan
Short-term Debt [Line Items]
Weighted average borrowings	$ 11.0
Highest borrowings outstanding	$ 250.0
Weighted average interest rate	0.80%
Borrowings outstanding	$ 250.0
Weighted average interest rate	0.80%
Spire Missouri \| Spire Note
Short-term Debt [Line Items]
Weighted average borrowings	$ 360.8
Lowest borrowings outstanding	277.2
Highest borrowings outstanding	$ 441.9
Weighted average interest rate	0.30%
Borrowings outstanding	$ 369.4	$ 301.2	$ 39.6
Weighted average interest rate	0.20%	0.20%	1.90%
Spire Missouri \| Revolving Credit Facility Borrowings
Short-term Debt [Line Items]
Borrowings outstanding			$ 185.1
Weighted average interest rate			1.80%
Spire
Short-term Debt [Line Items]
Weighted average borrowings	$ 681.4
Lowest borrowings outstanding	455.5
Highest borrowings outstanding	$ 850.5
Weighted average interest rate	0.40%
Borrowings outstanding	$ 653.5	$ 648.0	$ 560.6
Weighted average interest rate	0.40%	0.60%	1.90%
Spire \| Term Loan
Short-term Debt [Line Items]
Weighted average borrowings	$ 63.5
Highest borrowings outstanding	$ 150.0
Weighted average interest rate	1.10%
Borrowings outstanding		$ 150.0	$ 150.0
Weighted average interest rate		1.10%	2.10%
Spire \| Commercial Paper Notes
Short-term Debt [Line Items]
Weighted average borrowings	$ 606.9
Lowest borrowings outstanding	403.5
Highest borrowings outstanding	$ 775.0
Weighted average interest rate	0.30%
Borrowings outstanding	$ 403.5	$ 498.0	$ 175.5
Weighted average interest rate	0.20%	0.20%	1.80%

Minimum amount borrowed under the credit facility at any time during the period.

No definition available.

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

- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

+ Details

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

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

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

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

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

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, the maximum amount borrowed at any time during the period.

+ References

No definition available.

+ Details

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

- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=sr_SpireNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_LineOfCreditMember
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_LongtermDebtTypeAxis=sr_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: