Note 10 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Revenues from external customers	$ 1,123.4	$ 880.9	$ 1,937.4	$ 1,436.3
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	1,123.4	880.9	1,937.4	1,436.3
Natural gas	586.5	392.0	1,005.7	641.2
Operation and maintenance	132.1	113.2	264.2	229.6
Depreciation and amortization	62.6	58.9	124.7	115.8
Taxes, other than income taxes	81.9	71.6	132.3	109.2
Total Operating Expenses	863.1	635.7	1,526.9	1,095.8
Operating Income (Loss)	260.3	245.2	410.5	340.5
Net Economic Earnings (Loss)	199.2	181.0	284.3	243.6
Other Segments [Member]
Revenues from external customers	0.2	0.1	0.3	0.3
Intersegment revenues	3.9	4.2	7.8	7.7
Operating Revenues	4.1	4.3	8.1	8.0
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	4.9	4.2	8.9	8.4
Depreciation and amortization	0.0	0.1	0.2	0.2
Taxes, other than income taxes	0.1	0.0	0.1	0.0
Total Operating Expenses	5.0	4.3	9.2	8.6
Operating Income (Loss)	(0.9)	0.0	(1.1)	(0.6)
Net Economic Earnings (Loss)	(10.7)	(5.6)	(18.0)	(13.2)
Operating Segments [Member]
Revenues from external customers			0.0
Intersegment revenues			24.6
Operating Revenues			24.6
Natural gas			16.7
Operation and maintenance			7.9
Depreciation and amortization			0.0
Taxes, other than income taxes			0.0
Total Operating Expenses			24.6
Operating Income (Loss)			0.0
Net Economic Earnings (Loss)			0.0
Operating Segments [Member] | Gas Utility [Member]
Revenues from external customers	1,054.4	817.4	1,787.4	1,320.9
Intersegment revenues	0.1	0.0	0.1	0.0
Operating Revenues	1,054.5	817.4	1,787.5	1,320.9
Natural gas	543.3	356.0	944.9	566.2
Operation and maintenance	119.3	104.2	239.2	211.5
Depreciation and amortization	60.2	56.5	119.9	111.1
Taxes, other than income taxes	80.4	70.3	130.3	107.3
Total Operating Expenses	803.2	587.0	1,434.3	996.1
Operating Income (Loss)	251.3	230.4	353.2	324.8
Net Economic Earnings (Loss)	183.9	169.2	246.8	236.4
Operating Segments [Member] | Gas Marketing [Member]
Revenues from external customers	60.6	59.4	134.7	107.3
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	60.6	59.4	134.7	107.3
Natural gas	51.5	45.8	77.5	93.8
Operation and maintenance	5.7	3.2	12.0	5.9
Depreciation and amortization	0.4	0.4	0.7	0.7
Taxes, other than income taxes	0.6	0.4	0.7	0.4
Total Operating Expenses	58.2	49.8	90.9	100.8
Operating Income (Loss)	2.4	9.6	43.8	6.5
Net Economic Earnings (Loss)	21.8	14.4	47.5	14.9
Operating Segments [Member] | Midstream [Member]
Revenues from external customers	8.2	4.0	15.0	7.8
Intersegment revenues	8.3	9.8	16.7	18.9
Operating Revenues	16.5	13.8	31.7	26.7
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	6.2	5.8	12.0	11.6
Depreciation and amortization	2.0	1.9	3.9	3.8
Taxes, other than income taxes	0.8	0.9	1.2	1.5
Total Operating Expenses	9.0	8.6	17.1	16.9
Operating Income (Loss)	7.5	5.2	14.6	9.8
Net Economic Earnings (Loss)	4.2	3.0	$ 8.0	5.5
Consolidation, Eliminations [Member]
Revenues from external customers	0.0	0.0		0.0
Intersegment revenues	(12.3)	(14.0)		(26.6)
Operating Revenues	(12.3)	(14.0)		(26.6)
Natural gas	(8.3)	(9.8)		(18.8)
Operation and maintenance	(4.0)	(4.2)		(7.8)
Depreciation and amortization	0.0	0.0		0.0
Taxes, other than income taxes	0.0	0.0		0.0
Total Operating Expenses	(12.3)	(14.0)		(26.6)
Operating Income (Loss)	0.0	0.0		0.0
Net Economic Earnings (Loss)	$ 0.0	$ 0.0		$ 0.0