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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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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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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020 USD ($) Station	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 reporting_unit	Jun. 30, 2020 operating-segment	Jun. 30, 2019 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of reportable segments				2	2
Impairment charge of long-lived assets	$ 148.6	$ 0.0	$ 148.6			$ 0.0
Spire Storage West LLC ("Spire Storage") \| Non-Utility Property
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impairment charge of long-lived assets	$ 140.8
Number of commercial stations impaired \| Station	2
Spire Storage West LLC ("Spire Storage") \| Two Compressed Natural Gas Fueling Stations (Non-utility Property)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impairment charge of long-lived assets	$ 7.8
Spire Alabama \| Affiliated Entity \| Spire Marketing \| Regulated Operation
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Purchases of natural gas from related party	0.8		$ 5.1
Sales of natural gas to Spire Marketing Inc.	$ 0.3