Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - Spire Missouri - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Spire Marketing \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from related party	$ 56.9	$ 95.3	$ 71.5
Sales of natural gas to Spire Marketing Inc.	0.1	1.7	0.3
Spire STL Pipeline LLC \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	27.9
Spire NGL Inc. \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	$ 1.0	$ 1.0	$ 1.0

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

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-Name Accounting Standards Codification
-Topic 850
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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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