Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Revenues from external customers	$ 669.1	$ 756.6
Intersegment revenues	0.0	0.0
Total Operating Revenues	669.1	756.6
Depreciation and amortization expense	72.3	67.0
Interest expense	48.0	50.6
Income tax expense (benefit)	20.1	21.0
Adjusted earnings (loss)	81.1	82.7
Capital expenditures	260.6	226.5
Other Operating Segment [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	0.3	0.1
Intersegment revenues	4.4	4.0
Total Operating Revenues	4.7	4.1
Depreciation and amortization expense	0.1	0.1
Interest expense	24.8	24.0
Income tax expense (benefit)	(0.6)	0.9
Adjusted earnings (loss)	(10.9)	(2.7)
Capital expenditures	0.0	0.0
Operating Segments [Member] | Gas Utility [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	613.5	715.2
Intersegment revenues	0.0	0.0
Total Operating Revenues	613.5	715.2
Depreciation and amortization expense	68.1	64.2
Interest expense	35.1	39.0
Income tax expense (benefit)	16.1	15.7
Adjusted earnings (loss)	77.8	75.8
Capital expenditures	217.9	174.9
Operating Segments [Member] | Gas Marketing [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	33.0	36.3
Intersegment revenues	0.0	0.0
Total Operating Revenues	33.0	36.3
Depreciation and amortization expense	0.4	0.4
Interest expense	0.0	0.0
Income tax expense (benefit)	0.8	3.8
Adjusted earnings (loss)	2.2	7.2
Capital expenditures	0.1	0.0
Operating Segments [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	22.3	5.0
Intersegment revenues	11.2	9.9
Total Operating Revenues	33.5	14.9
Depreciation and amortization expense	3.7	2.3
Interest expense	1.2	1.8
Income tax expense (benefit)	3.8	0.6
Adjusted earnings (loss)	12.0	2.4
Capital expenditures	42.7	52.0
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	0.0	0.0
Intersegment revenues	(15.6)	(13.9)
Total Operating Revenues	(15.6)	(13.9)
Depreciation and amortization expense	0.0	0.0
Interest expense	(13.1)	(14.2)
Income tax expense (benefit)	0.0	0.0
Adjusted earnings (loss)	0.0	0.0
Capital expenditures	$ (0.1)	$ (0.4)