Note 10 - Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Total Assets	$ 10,037.5	$ 10,083.7	$ 9,783.9
Operating Segments [Member] | Gas Utility [Member]
Total Assets	8,283.2	8,042.8	7,854.7
Operating Segments [Member] | Gas Marketing [Member]
Total Assets	316.6	638.7	595.5
Operating Segments [Member] | Midstream [Member]
Total Assets	533.3	446.0	430.9
Operating Segments [Member] | Other Segments [Member]
Total Assets	2,134.1	2,449.2	2,222.0
Consolidation, Eliminations [Member]
Total Assets	$ (1,229.7)	$ (1,493.0)	$ (1,319.2)