Note 10 - Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023
Total Assets	$ 10,711.4	$ 10,313.6	$ 10,049.0
Operating Segments [Member] | Gas Utility [Member]
Total Assets	8,646.3	8,486.7	8,317.6
Operating Segments [Member] | Gas Marketing [Member]
Total Assets	191.4	332.0	335.0
Operating Segments [Member] | Midstream [Member]
Total Assets	845.2	574.3	506.2
Operating Segments [Member] | Other Operating Segment [Member]
Total Assets	2,467.2	2,533.3	1,959.1
Consolidation, Eliminations [Member]
Total Assets	$ (1,438.7)	$ (1,612.7)	$ (1,068.9)