Note 10 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Revenues from external customers	$ 756.6	$ 814.0
Intersegment revenues	0.0	0.0
Operating Revenues	756.6	814.0
Depreciation and amortization	67.0	62.1
Interest Expense, Net	50.6	43.6
Income Tax Expense	21.0	21.6
Net economic earnings (loss)	82.7	85.1
Capital expenditures	226.5	154.8
Consolidation, Eliminations [Member]
Revenues from external customers	0.0	0.0
Intersegment revenues	(13.9)	(12.3)
Operating Revenues	(13.9)	(12.3)
Depreciation and amortization	0.0	0.0
Interest Expense, Net	(14.2)	(9.9)
Income Tax Expense	0.0	0.0
Net economic earnings (loss)	0.0	0.0
Capital expenditures	(0.4)	0.0
Gas Utility [Member] | Operating Segments [Member]
Revenues from external customers	715.2	733.0
Intersegment revenues	0.0	0.0
Operating Revenues	715.2	733.0
Depreciation and amortization	64.2	59.7
Interest Expense, Net	39.0	32.1
Income Tax Expense	15.7	12.4
Net economic earnings (loss)	75.8	62.9
Capital expenditures	174.9	146.3
Gas Marketing [Member] | Operating Segments [Member]
Revenues from external customers	36.3	74.1
Intersegment revenues	0.0	0.0
Operating Revenues	36.3	74.1
Depreciation and amortization	0.4	0.3
Interest Expense, Net	0.0	0.0
Income Tax Expense	3.8	10.5
Net economic earnings (loss)	7.2	25.7
Capital expenditures	0.0	0.2
Midstream [Member] | Operating Segments [Member]
Revenues from external customers	5.0	6.8
Intersegment revenues	9.9	8.4
Operating Revenues	14.9	15.2
Depreciation and amortization	2.3	1.9
Interest Expense, Net	1.8	1.9
Income Tax Expense	0.6	1.4
Net economic earnings (loss)	2.4	3.8
Capital expenditures	52.0	7.7
Other Segments [Member]
Depreciation and amortization	0.1	0.2
Interest Expense, Net	24.0	19.5
Income Tax Expense	0.9	(2.7)
Other Segments [Member] | Consolidation, Eliminations [Member]
Revenues from external customers	0.1	0.1
Intersegment revenues	4.0	3.9
Operating Revenues	4.1	4.0
Net economic earnings (loss)	(2.7)	(7.3)
Capital expenditures	$ 0.0	$ 0.6