Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenues from external customers	$ 1,051.3	$ 1,128.5	$ 1,720.4	$ 1,885.1
Intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	1,051.3	1,128.5	1,720.4	1,885.1
Depreciation and amortization expense	73.7	68.9	146.0	135.9
Interest expense	47.4	52.2	95.4	102.8
Income tax expense (benefit)	52.7	49.4	72.8	70.4
Adjusted earnings (loss)	214.4	196.6	295.5	279.3
Capital expenditures	218.6	182.8	479.2	409.3
Other Operating Segment [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	0.5	0.1	0.8	0.2
Intersegment revenues	4.4	4.0	8.8	8.0
Total Operating Revenues	4.9	4.1	9.6	8.2
Depreciation and amortization expense	0.1	0.1	0.2	0.2
Interest expense	24.5	22.3	49.3	46.3
Income tax expense (benefit)	(0.7)	(1.4)	(1.3)	(0.5)
Adjusted earnings (loss)	(11.4)	(10.7)	(22.3)	(13.4)
Capital expenditures	0.0	0.0	0.0	0.0
Operating Segments [Member] | Gas Utility [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	970.1	1,072.4	1,583.6	1,787.6
Intersegment revenues	0.1	0.3	0.1	0.3
Total Operating Revenues	970.2	1,072.7	1,583.7	1,787.9
Depreciation and amortization expense	69.5	65.4	137.6	129.6
Interest expense	33.8	38.3	68.9	77.3
Income tax expense (benefit)	45.2	41.7	61.3	57.4
Adjusted earnings (loss)	195.2	188.0	273.0	263.8
Capital expenditures	177.8	137.3	395.7	312.2
Operating Segments [Member] | Gas Marketing [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	53.6	46.0	86.6	82.3
Intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	53.6	46.0	86.6	82.3
Depreciation and amortization expense	0.3	0.4	0.7	0.8
Interest expense	0.0	0.0	0.0	0.0
Income tax expense (benefit)	3.3	7.6	4.1	11.4
Adjusted earnings (loss)	14.8	15.5	17.0	22.7
Capital expenditures	0.0	0.0	0.1	0.0
Operating Segments [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	27.1	10.0	49.4	15.0
Intersegment revenues	11.3	11.5	22.5	21.4
Total Operating Revenues	38.4	21.5	71.9	36.4
Depreciation and amortization expense	3.8	3.0	7.5	5.3
Interest expense	1.3	2.0	2.5	3.8
Income tax expense (benefit)	4.9	1.5	8.7	2.1
Adjusted earnings (loss)	15.8	3.8	27.8	6.2
Capital expenditures	41.3	46.0	84.0	98.0
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	0.0	0.0	0.0	0.0
Intersegment revenues	(15.8)	(15.8)	(31.4)	(29.7)
Total Operating Revenues	(15.8)	(15.8)	(31.4)	(29.7)
Depreciation and amortization expense	0.0	0.0	0.0	0.0
Interest expense	(12.2)	(10.4)	(25.3)	(24.6)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Adjusted earnings (loss)	0.0	0.0	0.0	0.0
Capital expenditures	$ (0.5)	$ (0.5)	$ (0.6)	$ (0.9)