NOTES PAYABLE AND CREDIT AGREEMENTS - Spire Short-term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 408.6
Weighted average interest rate	2.00%
Minimum borrowing outstanding	$ 146.0
Maximum borrowing outstanding	632.9
Borrowings outstanding	$ 553.6	$ 477.3
Weighted average interest rate	2.40%	1.50%
Commercial Paper Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 408.5
Weighted average interest rate	2.00%
Minimum borrowing outstanding	$ 146.0
Maximum borrowing outstanding	632.9
Borrowings outstanding	$ 553.6	$ 477.3
Weighted average interest rate	2.40%	1.50%
Revolving Credit Facility Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 0.1
Weighted average interest rate	2.80%
Minimum borrowing outstanding	$ 0.0
Maximum borrowing outstanding	25.0
Borrowings outstanding	$ 0.0	$ 0.0
Weighted average interest rate	0.00%	0.00%

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

No definition available.

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Namespace Prefix:	lg_
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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=114775744&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=6877327&loc=d3e13212-122682

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- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- 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
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
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-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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

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- Definition

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

+ References

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- 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.

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