Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 3.5	$ 2.2
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	0.0	4.9
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	8.2	8.1
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 1.8	$ 1.8