Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - Spire Missouri - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Spire Marketing Inc \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from related party	$ 10.7	$ 19.5
Sales of natural gas to Spire Marketing Inc.	1.1
Spire STL Pipeline LLC \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	8.1	3.9
Spire NGL Inc. \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	$ 0.3	$ 0.3

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
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-Section 50
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
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-Name Accounting Standards Codification
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