Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - Spire Missouri - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Spire Marketing Inc \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing Inc.	$ 19.5	$ 39.8
Sales of natural gas to Spire Marketing Inc.		1.4
Spire STL Pipeline LLC \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received from	3.9
Spire NGL Inc. \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received from	$ 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
-SubTopic 10
-Section 50
-Paragraph 1
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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=116637345&loc=SL114874292-224272

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-Name Accounting Standards Codification
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