Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 3.0	$ 0.0	$ 11.2	$ 10.6
Transaction amount	0.0	1.4	0.1	1.7
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	0.0	0.0	13.9	3.4
Unregulated Operation [Member] \| Spire S T L Pipeline L L C [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	8.0	7.9	23.9	24.4
Unregulated Operation [Member] \| Spire MoGas Pipeline [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	1.6	1.7	5.3	3.1
Unregulated Operation [Member] \| Spire Storage Salt Plains LLC [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 0.0	$ 0.0	$ 0.0	$ 0.7