Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024
Segment Reporting Information [Line Items]
Total Assets	$ 11,346.7	$ 10,860.7	$ 10,711.4
Operating Segments [Member] | Gas Utility [Member]
Segment Reporting Information [Line Items]
Total Assets	9,140.5	8,767.2	8,646.3
Operating Segments [Member] | Gas Marketing [Member]
Segment Reporting Information [Line Items]
Total Assets	276.9	206.1	191.4
Operating Segments [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Total Assets	963.2	903.4	845.2
Operating Segments [Member] | Other Operating Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	2,736.9	2,689.8	2,467.2
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Total Assets	$ (1,770.8)	$ (1,705.8)	$ (1,438.7)