Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Spire Missouri \| Spire Marketing Inc \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from related party	$ 18.2	$ 10.7
Sales of natural gas to Spire Marketing Inc.		1.1
Spire Missouri \| Spire STL Pipeline LLC \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received	$ 8.1	8.1
Spire Missouri \| Spire NGL Inc. \| Unregulated Operation
Related Party Transaction [Line Items]
Transportation services received		0.3
Spire Alabama \| Spire Marketing Inc \| Regulated Operation
Related Party Transaction [Line Items]
Purchases of natural gas from related party		$ 4.8

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

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