Note 10 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Revenues from external customers	$ 1,128.5	$ 1,123.4	$ 1,885.1	$ 1,937.4
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	1,128.5	1,123.4	1,885.1	1,937.4
Depreciation and amortization	68.9	62.6	135.9	124.7
Interest Expense, Net	52.2	47.2	102.8	90.8
Income Tax Expense	49.4	40.9	70.4	62.5
Net economic earnings (loss)	196.6	199.2	279.3	284.3
Capital expenditures	182.8	153.0	409.3	307.8
Consolidation, Eliminations [Member]
Revenues from external customers	0.0	0.0	0.0	0.0
Intersegment revenues	(15.8)	(12.3)	(29.7)	(24.6)
Operating Revenues	(15.8)	(12.3)	(29.7)	(24.6)
Depreciation and amortization	0.0	0.0	0.0	0.0
Interest Expense, Net	(10.4)	(7.2)	(24.6)	(17.1)
Income Tax Expense	0.0	0.0	0.0	0.0
Net economic earnings (loss)	0.0	0.0	0.0	0.0
Capital expenditures	(0.5)	0.0	(0.9)	0.0
Gas Utility [Member] | Operating Segments [Member]
Revenues from external customers	1,072.4	1,054.4	1,787.6	1,787.4
Intersegment revenues	0.3	0.1	0.3	0.1
Operating Revenues	1,072.7	1,054.5	1,787.9	1,787.5
Depreciation and amortization	65.4	60.2	129.6	119.9
Interest Expense, Net	38.3	35.5	77.3	67.6
Income Tax Expense	41.7	38.9	57.4	51.3
Net economic earnings (loss)	188.0	183.9	263.8	246.8
Capital expenditures	137.3	143.7	312.2	290.0
Gas Marketing [Member] | Operating Segments [Member]
Revenues from external customers	46.0	60.6	82.3	134.7
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	46.0	60.6	82.3	134.7
Depreciation and amortization	0.4	0.4	0.8	0.7
Interest Expense, Net	0.0	0.0	0.0	0.0
Income Tax Expense	7.6	0.8	11.4	11.3
Net economic earnings (loss)	15.5	21.8	22.7	47.5
Capital expenditures	0.0	0.0	0.0	0.2
Midstream [Member] | Operating Segments [Member]
Revenues from external customers	10.0	8.2	15.0	15.0
Intersegment revenues	11.5	8.3	21.4	16.7
Operating Revenues	21.5	16.5	36.4	31.7
Depreciation and amortization	3.0	2.0	5.3	3.9
Interest Expense, Net	2.0	1.9	3.8	3.8
Income Tax Expense	1.5	1.5	2.1	2.9
Net economic earnings (loss)	3.8	4.2	6.2	8.0
Capital expenditures	46.0	9.3	98.0	17.0
Other Operating Segment [Member]
Depreciation and amortization	0.1	0.0	0.2	0.2
Interest Expense, Net	22.3	17.0	46.3	36.5
Income Tax Expense	(1.4)	(0.3)	(0.5)	(3.0)
Other Operating Segment [Member] | Consolidation, Eliminations [Member]
Revenues from external customers	0.1	0.2	0.2	0.3
Intersegment revenues	4.0	3.9	8.0	7.8
Operating Revenues	4.1	4.1	8.2	8.1
Net economic earnings (loss)	(10.7)	(10.7)	(13.4)	(18.0)
Capital expenditures	$ 0.0	$ 0.0	$ 0.0	$ 0.6