Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 2.2	$ 5.0
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	4.9	3.4
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	8.1	8.2
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	1.8	0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 0.0	$ 0.3