Note 14 - Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Total Assets	$ 10,083.7	$ 9,356.4	$ 8,241.2
Operating Segments [Member] | Gas Utility [Member]
Total Assets	8,042.8	7,615.4	6,716.2
Operating Segments [Member] | Gas Marketing [Member]
Total Assets	638.7	466.1	182.7
Operating Segments [Member] | Other Segments [Member]
Total Assets	2,895.2	2,351.7	2,443.5
Consolidation, Eliminations [Member]
Total Assets	$ (1,493.0)	$ (1,076.8)	$ (1,101.2)