SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	12 Months Ended
Sep. 30, 2017 USD ($) operating_segment change	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Related Party Transaction [Line Items]
Number of operating segments \| operating_segment	2
Utility Plant, Depreciation and Amortization [Abstract]
Accruals for capital expenditures	$ 41,000,000	$ 30,400,000	$ 13,400,000
Amount by which the replacement cost exceeds (less than) the cost basis value of stored gas inventories	$ 20,800,000	11,400,000
Number of months of the original maturity of debt instruments purchased	3 months
Finance receivable, net	$ 12,500,000	11,800,000
Retrospective Effect is ASU had been Adopted \| ASU 2015-03 \| New Accounting Pronouncement, Early Adoption, Effect \| Long-Term Debt
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs	$ 13,200,000	$ 13,000,000
Spire Missouri
Utility Plant, Depreciation and Amortization [Abstract]
Average annual depreciation and amortization expense percentage	3.00%	3.00%	3.00%
Accruals for capital expenditures	$ 28,900,000	$ 14,800,000	$ 9,600,000
Number of PGA discretionary changes \| change	3
Months between PGA discretionary changes	2 months
Cost savings retention percentage	10.00%
Maximum amount of cost savings allowable	$ 3,000,000.0
Accrued unbilled revenues	30,100,000	26,100,000
Spire Missouri \| Retrospective Effect is ASU had been Adopted \| ASU 2015-03 \| New Accounting Pronouncement, Early Adoption, Effect \| Long-Term Debt
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs	4,200,000	4,800,000
Spire Alabama
Utility Plant, Depreciation and Amortization [Abstract]
Accruals for capital expenditures	$ 9,400,000	6,800,000	$ 3,100,000
Composite depreciation rate	3.10%
Accrued unbilled revenues	$ 0	5,900,000
Threshold period past due for collection	90 days
Threshold period past due for write-off of financing receivable	12 months
Write off on an allowance for credit losses	$ 400,000	400,000
Spire Alabama \| Retrospective Effect is ASU had been Adopted \| ASU 2015-03 \| New Accounting Pronouncement, Early Adoption, Effect \| Long-Term Debt
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs	2,600,000	2,400,000
Spire Alabama \| Financing Receivables, Equal to Greater than 90 Days Past Due
Utility Plant, Depreciation and Amortization [Abstract]
Finance receivable 90 days past due	$ 400,000	$ 400,000

The average annual depreciation and amortization expense as stated as a percentage of original cost of property.

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

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The amount by which the replacement cost exceeds (is less than) the cost basis value of stored gas inventories.

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

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

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The number of months the original maturity of debt instruments purchased can not exceed to be considered cash equivalents.

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

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Threshold Period Past Due for Collection

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For an unclassified balance sheet, the amount of fees and other revenue, excluding investment income receivable, earned but not yet received, which were recognized in conformity with revenue recognition criteria based on estimates or specific contractual terms.

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Future cash outflow to pay for purchases of fixed assets that have occurred.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of direct write-downs of financing receivables charged against the allowance.

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Recorded Investment in financing receivables that are past due at the balance sheet date.

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Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

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- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Composite depreciation rate for public utility plants in service.

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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.

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Threshold period past due for financing receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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