Information by Operating Segment - Total Assets by Segment (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024
Segment Reporting Information [Line Items]
Total Assets	$ 11,396.3	$ 10,860.7	$ 10,710.9
Operating Segments [Member] \| Gas Utility [Member]
Segment Reporting Information [Line Items]
Total Assets	9,174.8	8,767.2	8,601.9
Operating Segments [Member] \| Gas Marketing [Member]
Segment Reporting Information [Line Items]
Total Assets	264.9	206.1	212.3
Operating Segments [Member] \| Midstream [Member]
Segment Reporting Information [Line Items]
Total Assets	970.9	903.4	871.9
Operating Segments [Member] \| Other Operating Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	2,687.1	2,689.8	2,627.4
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Total Assets	$ (1,701.4)	$ (1,705.8)	$ (1,602.6)

Amount of asset recognized for present right to economic benefit.

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+ Details