Note 14 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Revenues from external customers	$ 2,666.3	$ 2,198.5	$ 2,235.5
Intersegment revenues	0.0	0.0	0.0
Operating Revenues	2,666.3	2,198.5	2,235.5
Depreciation and amortization	254.8	237.3	213.1
Interest Expense	185.7	119.8	106.6
Income Tax Expense	38.8	58.9	68.5
Net economic earnings (loss)	228.1	216.3	266.3
Capital expenditures	662.5	552.2	624.8
Consolidation, Eliminations [Member]
Revenues from external customers	0.0	0.0	0.0
Intersegment revenues	(52.5)	(51.7)	(48.0)
Operating Revenues	(52.5)	(51.7)	(48.0)
Depreciation and amortization	0.0	0.0	0.0
Interest Expense	(34.9)	(8.8)	(1.4)
Income Tax Expense	0.0	0.0	0.0
Net economic earnings (loss)	0.0	0.0	0.0
Capital expenditures	0.0	0.0	0.0
Gas Utility [Member] | Operating Segments [Member]
Revenues from external customers	2,456.6	1,945.6	2,118.2
Intersegment revenues	0.3	0.5	1.1
Operating Revenues	2,456.9	1,946.1	2,119.3
Depreciation and amortization	244.4	227.9	204.4
Interest Expense	139.9	86.4	74.7
Income Tax Expense	32.7	49.6	56.9
Net economic earnings (loss)	200.5	202.7	230.6
Capital expenditures	588.6	528.6	590.4
Gas Marketing [Member] | Operating Segments [Member]
Revenues from external customers	179.1	234.9	96.5
Intersegment revenues	0.0	0.0	0.0
Operating Revenues	179.1	234.9	96.5
Depreciation and amortization	1.5	1.4	1.2
Interest Expense	0.0	0.0	0.0
Income Tax Expense	12.8	11.9	13.4
Net economic earnings (loss)	47.6	27.0	47.0
Capital expenditures	0.4	0.9	0.7
Midstream [Member] | Operating Segments [Member]
Revenues from external customers	29.9	17.4	19.9
Intersegment revenues	36.2	35.7	32.7
Operating Revenues	66.1	53.1	52.6
Depreciation and amortization	8.4	7.5	6.9
Interest Expense	8.6	4.9	4.0
Income Tax Expense	3.8	4.5	4.4
Net economic earnings (loss)	14.1	11.1	11.1
Capital expenditures	73.6	22.6	33.0
Other Segments [Member]
Depreciation and amortization	0.5	0.5	0.6
Interest Expense	72.1	37.3	29.3
Income Tax Expense	(10.5)	(7.1)	(6.2)
Other Segments [Member] | Consolidation, Eliminations [Member]
Revenues from external customers	0.7	0.6	0.9
Intersegment revenues	16.0	15.5	14.2
Operating Revenues	16.7	16.1	15.1
Net economic earnings (loss)	(34.1)	(24.5)	(22.4)
Capital expenditures	$ (0.1)	$ 0.1	$ 0.7