Note 10 - Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Total Assets	$ 10,631.7	$ 10,313.6	$ 10,327.7
Operating Segments [Member] | Gas Utility [Member]
Total Assets	8,736.7	8,486.7	8,355.1
Operating Segments [Member] | Gas Marketing [Member]
Total Assets	207.0	332.0	492.4
Operating Segments [Member] | Midstream [Member]
Total Assets	633.9	574.3	466.6
Operating Segments [Member] | Other Segments [Member]
Total Assets	2,755.1	2,533.3	2,882.8
Consolidation, Eliminations [Member]
Total Assets	$ (1,701.0)	$ (1,612.7)	$ (1,869.2)