Note 10 - Information by Operating Segment - Net Economic Earnings to Net Income (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
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	1,123.4	880.9	1,937.4	1,436.3
Total Operating Revenues	(1,123.4)	(880.9)	(1,937.4)	(1,436.3)
Natural gas	586.5	392.0	1,005.7	641.2
Natural gas	(586.5)	(392.0)	(1,005.7)	(641.2)
Operation and maintenance	132.1	113.2	264.2	229.6
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
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
Net Income	179.2	173.6	270.2	229.3
Fair value and timing adjustments	26.6	9.9	18.8	13.6
Income tax adjustments	(6.6)	(2.5)	(4.7)	0.7
Net Economic Earnings	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
Intersegment revenues	(3.9)	(4.2)	(7.8)	(7.7)
Operating Revenues	4.1	4.3	8.1	8.0
Total Operating Revenues	(4.1)	(4.3)	(8.1)	(8.0)
Natural gas	0.0	0.0	0.0	0.0
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	4.9	4.2	8.9	8.4
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
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)
Net Economic Earnings	(10.7)	(5.6)	(18.0)	(13.2)
Operating Segments [Member]
Revenues from external customers			0.0
Intersegment revenues			24.6
Intersegment revenues			(24.6)
Operating Revenues			24.6
Total Operating Revenues			(24.6)
Natural gas			16.7
Natural gas			(16.7)
Operation and maintenance			7.9
Operation and maintenance			(7.9)
Depreciation and amortization			0.0
Taxes, other than income taxes			0.0
Total Operating Expenses			24.6
Total Operating Expenses			(24.6)
Operating Income (Loss)			0.0
Net Economic Earnings (Loss)			0.0
Net Economic Earnings			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
Intersegment revenues	(0.1)	0.0	(0.1)	0.0
Operating Revenues	1,054.5	817.4	1,787.5	1,320.9
Total Operating Revenues	(1,054.5)	(817.4)	(1,787.5)	(1,320.9)
Natural gas	543.3	356.0	944.9	566.2
Natural gas	(543.3)	(356.0)	(944.9)	(566.2)
Operation and maintenance	119.3	104.2	239.2	211.5
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
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
Net Economic Earnings	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
Intersegment revenues	0.0	0.0	0.0	0.0
Operating Revenues	60.6	59.4	134.7	107.3
Total Operating Revenues	(60.6)	(59.4)	(134.7)	(107.3)
Natural gas	51.5	45.8	77.5	93.8
Natural gas	(51.5)	(45.8)	(77.5)	(93.8)
Operation and maintenance	5.7	3.2	12.0	5.9
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
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
Net Economic Earnings	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
Intersegment revenues	(8.3)	(9.8)	(16.7)	(18.9)
Operating Revenues	16.5	13.8	31.7	26.7
Total Operating Revenues	(16.5)	(13.8)	(31.7)	(26.7)
Natural gas	0.0	0.0	0.0	0.0
Natural gas	0.0	0.0	0.0	0.0
Operation and maintenance	6.2	5.8	12.0	11.6
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
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
Net Economic Earnings	$ 4.2	$ 3.0	$ 8.0	$ 5.5