Note 14 - Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Total Assets	$ 10,313.6	$ 10,083.7	$ 9,356.4
Operating Segments [Member] | Gas Utility [Member]
Total Assets	8,486.7	8,042.8	7,615.4
Operating Segments [Member] | Gas Marketing [Member]
Total Assets	332.0	638.7	466.1
Operating Segments [Member] | Midstream [Member]
Total Assets	574.3	446.0	413.8
Operating Segments [Member] | Other Segments [Member]
Total Assets	2,533.3	2,705.5	2,193.3
Consolidation, Eliminations [Member]
Total Assets	$ (1,612.7)	$ (1,749.3)	$ (1,332.2)