Note 10 - Information by Operating Segment - Net Economic Earnings to Net Income (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
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	418.5	448.0	2,355.9	1,884.3
Total Operating Revenues	(418.5)	(448.0)	(2,355.9)	(1,884.3)
Natural gas	169.8	203.3	1,175.5	844.5
Natural gas	(169.8)	(203.3)	(1,175.5)	(844.5)
Operation and maintenance	125.5	102.3	389.7	331.9
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
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
Net Income	(21.6)	(1.4)	248.6	227.9
Fair value and timing adjustments	3.4	7.3	22.2	20.9
Acquisition activities	0.5	0.0	0.5	0.0
Income tax adjustments	(0.9)	(1.8)	(5.6)	(1.1)
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
Intersegment revenues	(4.1)	(3.9)	(11.9)	(11.6)
Operating Revenues	4.3	4.1	12.4	12.1
Total Operating Revenues	(4.3)	(4.1)	(12.4)	(12.1)
Natural gas	0.0	0.0	0.0	0.0
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	5.5	3.2	14.4	11.6
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
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)
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
Intersegment revenues			(38.7)
Operating Revenues			38.7
Total Operating Revenues			(38.7)
Natural gas			26.8
Natural gas			(26.8)
Operation and maintenance			11.9
Operation and maintenance			(11.9)
Depreciation and amortization			0.0
Taxes, other than income taxes			0.0
Total Operating Expenses			38.7
Total Operating Expenses			(38.7)
Operating Income (Loss)			0.0
Net Economic (Loss) Earnings			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
Intersegment revenues	(0.3)	0.0	(0.4)	0.0
Operating Revenues	387.8	377.4	2,175.3	1,698.3
Total Operating Revenues	(387.8)	(377.4)	(2,175.3)	(1,698.3)
Natural gas	154.6	144.5	1,099.5	710.7
Natural gas	(154.6)	(144.5)	(1,099.5)	(710.7)
Operation and maintenance	111.7	95.0	350.9	306.5
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
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
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
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	23.1	64.1	157.8	171.4
Total Operating Revenues	(23.1)	(64.1)	(157.8)	(171.4)
Natural gas	25.3	67.1	102.8	160.9
Natural gas	(25.3)	(67.1)	(102.8)	(160.9)
Operation and maintenance	4.2	3.2	16.2	9.1
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
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
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
Intersegment revenues	(9.7)	(8.3)	(26.4)	(27.2)
Operating Revenues	17.4	14.6	49.1	41.3
Total Operating Revenues	(17.4)	(14.6)	(49.1)	(41.3)
Natural gas	0.0	0.0	0.0	0.0
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	8.1	4.8	20.1	16.4
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
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
Net Economic (Loss) Earnings	$ 3.6	$ 4.3	$ 11.6	$ 9.8