Financing Arrangements and Long-term Debt - Schedule of Spire Missouri and Alabama Short-term Borrowings (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Spire Missouri \| Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 313.5
Weighted average interest rate	2.80%
Minimum borrowing outstanding	$ 205.1
Maximum borrowing outstanding	404.9
Borrowings outstanding	$ 232.9	$ 345.3	$ 175.8
Weighted average interest rate	2.80%	2.30%	2.20%
Spire Alabama \| Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 117.3
Weighted average interest rate	2.80%
Minimum borrowing outstanding	$ 43.8
Maximum borrowing outstanding	169.2
Borrowings outstanding	$ 73.7	$ 142.5	$ 82.3
Weighted average interest rate	2.80%	2.30%	2.20%
Spire \| Commercial Paper Notes
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 598.3
Weighted average interest rate	2.80%
Minimum borrowing outstanding	$ 482.5
Maximum borrowing outstanding	689.3
Borrowings outstanding	$ 512.0	$ 553.6	$ 391.7
Weighted average interest rate	2.80%	2.40%	2.20%

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

No definition available.

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- 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
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- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

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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.

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- 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.

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- Definition

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

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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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