Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Total Assets	$ 11,275.8	$ 10,860.7	$ 10,631.7
Operating Segments [Member] | Gas Utility [Member]
Segment Reporting Information [Line Items]
Total Assets	9,075.2	8,767.2	8,736.7
Operating Segments [Member] | Gas Marketing [Member]
Segment Reporting Information [Line Items]
Total Assets	246.1	206.1	207.0
Operating Segments [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Total Assets	936.0	903.4	633.9
Operating Segments [Member] | Other Operating Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	2,872.8	2,689.8	2,755.1
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Total Assets	$ (1,854.3)	$ (1,705.8)	$ (1,701.0)