Note 10 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenues from external customers	$ 418.5	$ 448.0	$ 2,355.9	$ 1,884.3
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	418.5	448.0	2,355.9	1,884.3
Natural gas	169.8	203.3	1,175.5	844.5
Operation and maintenance	125.5	102.3	389.7	331.9
Depreciation and amortization	64.3	60.4	189.0	176.2
Taxes, other than income taxes	46.9	44.1	179.2	153.3
Total Operating Expenses	406.5	410.1	1,933.4	1,505.9
Operating Income (Loss)	12.0	37.9	422.5	378.4
Net Economic (Loss) Earnings	(18.6)	4.1	265.7	247.7
Other Segments [Member]
Revenues from external customers	0.2	0.2	0.5	0.5
Intersegment revenues	4.1	3.9	11.9	11.6
Operating Revenues	4.3	4.1	12.4	12.1
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	5.5	3.2	14.4	11.6
Depreciation and amortization	0.2	0.2	0.4	0.4
Taxes, other than income taxes	(0.1)	0.1	0.0	0.1
Total Operating Expenses	5.6	3.5	14.8	12.1
Operating Income (Loss)	(1.3)	0.6	(2.4)	0.0
Net Economic (Loss) Earnings	(7.4)	(4.8)	(25.4)	(18.0)
Operating Segments [Member]
Revenues from external customers			0.0
Intersegment revenues			38.7
Operating Revenues			38.7
Natural gas			26.8
Operation and maintenance			11.9
Depreciation and amortization			0.0
Taxes, other than income taxes			0.0
Total Operating Expenses			38.7
Operating Income (Loss)			0.0
Net Economic (Loss) Earnings			0.0
Operating Segments [Member] | Gas Utility [Member]
Revenues from external customers	387.5	377.4	2,174.9	1,698.3
Intersegment revenues	0.3	0.0	0.4	0.0
Operating Revenues	387.8	377.4	2,175.3	1,698.3
Natural gas	154.6	144.5	1,099.5	710.7
Operation and maintenance	111.7	95.0	350.9	306.5
Depreciation and amortization	61.7	58.1	181.6	169.2
Taxes, other than income taxes	45.9	43.0	176.2	150.3
Total Operating Expenses	373.9	340.6	1,808.2	1,336.7
Operating Income (Loss)	13.9	36.8	367.1	361.6
Net Economic (Loss) Earnings	(12.3)	4.2	234.5	240.6
Operating Segments [Member] | Gas Marketing [Member]
Revenues from external customers	23.1	64.1	157.8	171.4
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	23.1	64.1	157.8	171.4
Natural gas	25.3	67.1	102.8	160.9
Operation and maintenance	4.2	3.2	16.2	9.1
Depreciation and amortization	0.3	0.3	1.0	1.0
Taxes, other than income taxes	0.4	0.4	1.1	0.8
Total Operating Expenses	30.2	71.0	121.1	171.8
Operating Income (Loss)	(7.1)	(6.9)	36.7	(0.4)
Net Economic (Loss) Earnings	(2.5)	0.4	45.0	15.3
Operating Segments [Member] | Midstream [Member]
Revenues from external customers	7.7	6.3	22.7	14.1
Intersegment revenues	9.7	8.3	26.4	27.2
Operating Revenues	17.4	14.6	49.1	41.3
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	8.1	4.8	20.1	16.4
Depreciation and amortization	2.1	1.8	6.0	5.6
Taxes, other than income taxes	0.7	0.6	1.9	2.1
Total Operating Expenses	10.9	7.2	28.0	24.1
Operating Income (Loss)	6.5	7.4	21.1	17.2
Net Economic (Loss) Earnings	3.6	4.3	$ 11.6	9.8
Consolidation, Eliminations [Member]
Revenues from external customers	0.0	0.0		0.0
Intersegment revenues	(14.1)	(12.2)		(38.8)
Operating Revenues	(14.1)	(12.2)		(38.8)
Natural gas	(10.1)	(8.3)		(27.1)
Operation and maintenance	(4.0)	(3.9)		(11.7)
Depreciation and amortization	0.0	0.0		0.0
Taxes, other than income taxes	0.0	0.0		0.0
Total Operating Expenses	(14.1)	(12.2)		(38.8)
Operating Income (Loss)	0.0	0.0		0.0
Net Economic (Loss) Earnings	$ 0.0	$ 0.0		$ 0.0