Note 1 - Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Purchases of natural gas from Spire Marketing	$ 5.6	$ 13.8	$ 10.6	$ 42.3
Transaction amount	0.3	0.1	0.3	0.1
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Purchases of natural gas from Spire Marketing	0.0	0.0	3.4	1.1
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Transaction amount	8.3	7.8	16.5	15.9
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Missouri [Member]
Transaction amount	1.4	0.0	1.4	0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member]
Transaction amount	$ 0.4	$ 0.0	$ 0.7	$ 0.0