Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 6.0	$ 5.6	$ 8.2	$ 10.6
Transaction amount	0.1	0.3	0.1	0.3
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	9.0	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	7.8	8.3	15.9	16.5
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	1.9	1.4	3.7	1.4
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 0.0	$ 0.4	$ 0.0	$ 0.7