Note 14 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Revenues from external customers	$ 2,593.0	$ 2,666.3	$ 2,198.5
Intersegment revenues	0.0	0.0	0.0
Operating Revenues	2,593.0	2,666.3	2,198.5
Depreciation and amortization	278.4	254.8	237.3
Interest Expense	201.1	185.7	119.8
Income Tax Expense	58.7	38.8	58.9
Adjusted earnings (loss)	247.4	228.1	216.3
Capital expenditures	861.3	662.5	552.2
Consolidation, Eliminations [Member]
Revenues from external customers	0.0	0.0	0.0
Intersegment revenues	(62.4)	(52.5)	(51.7)
Operating Revenues	(62.4)	(52.5)	(51.7)
Depreciation and amortization	0.0	0.0	0.0
Interest Expense	(46.3)	(34.9)	(8.8)
Income Tax Expense	0.0	0.0	0.0
Adjusted earnings (loss)	0.0	0.0	0.0
Capital expenditures	(1.2)	0.0	0.0
Gas Utility [Member] | Operating Segments [Member]
Revenues from external customers	2,436.2	2,456.6	1,945.6
Intersegment revenues	1.7	0.3	0.5
Operating Revenues	2,437.9	2,456.9	1,946.1
Depreciation and amortization	263.6	244.4	227.9
Interest Expense	147.3	139.9	86.4
Income Tax Expense	48.0	32.7	49.6
Adjusted earnings (loss)	220.8	200.5	202.7
Capital expenditures	691.1	588.6	528.6
Gas Marketing [Member] | Operating Segments [Member]
Revenues from external customers	99.2	179.1	234.9
Intersegment revenues	0.0	0.0	0.0
Operating Revenues	99.2	179.1	234.9
Depreciation and amortization	1.5	1.5	1.4
Interest Expense	0.0	0.0	0.0
Income Tax Expense	10.8	12.8	11.9
Adjusted earnings (loss)	23.4	47.6	27.0
Capital expenditures	0.1	0.4	0.9
Midstream [Member] | Operating Segments [Member]
Revenues from external customers	56.9	29.9	17.4
Intersegment revenues	43.8	36.2	35.7
Operating Revenues	100.7	66.1	53.1
Depreciation and amortization	12.8	8.4	7.5
Interest Expense	7.0	8.6	4.9
Income Tax Expense	9.6	3.8	4.5
Adjusted earnings (loss)	33.5	14.1	11.1
Capital expenditures	171.3	73.6	22.6
Other Operating Segment [Member]
Depreciation and amortization	0.5	0.5	0.5
Interest Expense	93.1	72.1	37.3
Income Tax Expense	(9.7)	(10.5)	(7.1)
Adjusted earnings (loss)	(30.3)	(34.1)	(24.5)
Other Operating Segment [Member] | Consolidation, Eliminations [Member]
Revenues from external customers	0.7	0.7	0.6
Intersegment revenues	16.9	16.0	15.5
Operating Revenues	17.6	16.7	16.1
Capital expenditures	$ 0.0	$ (0.1)	$ 0.1