Note 9 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Revenues from external customers	$ 814.0		$ 555.4
Intersegment revenues	0.0		0.0
Operating Revenues	814.0	$ 814.0	555.4	$ 555.4
Natural gas	419.2	419.2	249.2	249.2
Operation and maintenance	132.1	132.1	116.4	116.4
Depreciation and amortization	62.1	62.1	56.9	56.9
Taxes, other than income taxes	50.4	50.4	37.6	37.6
Total Operating Expenses	663.8	663.8	460.1	460.1
Operating Income (Loss)	150.2	$ 150.2	95.3	$ 95.3
Net Economic Earnings (Loss)	85.1		62.6
Other Segments [Member]
Revenues from external customers	0.1		0.2
Intersegment revenues	3.9		3.5
Operating Revenues	4.0		3.7
Natural gas	0.0		0.0
Operation and maintenance	4.0		4.2
Depreciation and amortization	0.2		0.1
Taxes, other than income taxes	0.0		0.0
Total Operating Expenses	4.2		4.3
Operating Income (Loss)	(0.2)		(0.6)
Net Economic Earnings (Loss)	(7.3)		(7.6)
Operating Segments [Member] | Gas Utility [Member]
Revenues from external customers	733.0		503.5
Intersegment revenues	0.0		0.0
Operating Revenues	733.0		503.5
Natural gas	401.6		210.2
Operation and maintenance	119.9		107.3
Depreciation and amortization	59.7		54.6
Taxes, other than income taxes	49.9		37.0
Total Operating Expenses	631.1		409.1
Operating Income (Loss)	101.9		94.4
Net Economic Earnings (Loss)	62.9		67.2
Operating Segments [Member] | Gas Marketing [Member]
Revenues from external customers	74.1		47.9
Intersegment revenues	0.0		0.0
Operating Revenues	74.1		47.9
Natural gas	26.0		48.0
Operation and maintenance	6.3		2.7
Depreciation and amortization	0.3		0.3
Taxes, other than income taxes	0.1		0.0
Total Operating Expenses	32.7		51.0
Operating Income (Loss)	41.4		(3.1)
Net Economic Earnings (Loss)	25.7		0.5
Operating Segments [Member] | Midstream [Member]
Revenues from external customers	6.8		3.8
Intersegment revenues	8.4		9.1
Operating Revenues	15.2		12.9
Natural gas	0.0		0.0
Operation and maintenance	5.8		5.8
Depreciation and amortization	1.9		1.9
Taxes, other than income taxes	0.4		0.6
Total Operating Expenses	8.1		8.3
Operating Income (Loss)	7.1		4.6
Net Economic Earnings (Loss)	3.8		2.5
Consolidation, Eliminations [Member]
Revenues from external customers	0.0		0.0
Intersegment revenues	(12.3)		(12.6)
Operating Revenues	(12.3)		(12.6)
Natural gas	(8.4)		(9.0)
Operation and maintenance	(3.9)		(3.6)
Depreciation and amortization	0.0		0.0
Taxes, other than income taxes	0.0		0.0
Total Operating Expenses	(12.3)		(12.6)
Operating Income (Loss)	0.0		0.0
Net Economic Earnings (Loss)	$ 0.0		$ 0.0