Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Spire Marketing Inc \| Spire Missouri \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from related party	$ 57.6	$ 13.3	$ 68.3	$ 32.8
Sales of natural gas to Spire Marketing Inc.			1.1
Spire Marketing Inc \| Spire Alabama \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from related party	5.8	1.0	10.6	4.3
Sales of natural gas to Spire Marketing Inc.	0.1		0.1
Spire STL Pipeline LLC \| Spire Missouri \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	7.9	7.9	16.0	11.8
Spire NGL Inc. \| Spire Missouri \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	$ 0.2	$ 0.2	$ 0.5	$ 0.5

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-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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-URI http://asc.fasb.org/extlink&oid=120401555&loc=SL114874292-224272

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-Name Accounting Standards Codification
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