Note 9 - Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021	Jun. 30, 2021
Total Assets	$ 9,783.9	$ 9,356.4	$ 8,893.3
Operating Segments [Member] | Gas Utility [Member]
Total Assets	7,854.7	7,615.4	7,302.4
Operating Segments [Member] | Gas Marketing [Member]
Total Assets	595.5	466.1	349.2
Operating Segments [Member] | Other Segments [Member]
Total Assets	2,652.9	2,351.7	2,256.3
Consolidation, Eliminations [Member]
Total Assets	$ (1,319.2)	$ (1,076.8)	$ (1,014.6)