Note 9 - Information by Operating Segment - Segment Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues from external customers	$ 448,000	$ 327,800	$ 1,884,300	$ 1,945,300
Intersegment revenues	0	0	0	0
Operating Revenues	448,000	327,800	1,884,300	1,945,300
Natural gas	203,300	96,900	844,500	897,200
Operation and maintenance	102,300	112,000	331,900	342,600
Depreciation and amortization	60,400	53,100	176,200	155,400
Taxes, other than income taxes	44,100	32,600	153,300	126,600
Total Operating Expenses	410,100	294,600	1,505,900	1,521,800
Operating Income (Loss)	37,900	33,200	378,400	423,500
Net Economic Earnings (Loss)	4,100	6,900	247,700	279,400
Consolidation, Eliminations [Member]
Revenues from external customers	0	0	0	0
Intersegment revenues	(12,200)		(38,800)	(36,100)
Operating Revenues	(12,200)		(38,800)	(36,100)
Natural gas	(8,300)		(27,100)	(26,000)
Operation and maintenance	(3,900)	(3,600)	(11,700)	(10,100)
Depreciation and amortization	0	0	0	0
Taxes, other than income taxes	0	0	0	0
Total Operating Expenses	(12,200)		(38,800)	(36,100)
Operating Income (Loss)	0	0	0	0
Net Economic Earnings (Loss)	0	0	0	0
Segment Reconciling Items [Member]
Intersegment revenues		(11,800)
Operating Revenues		(11,800)
Natural gas		(8,200)
Total Operating Expenses		(11,800)
Gas Utility [Member] | Operating Segments [Member]
Revenues from external customers	377,400	306,700	1,698,300	1,856,900
Intersegment revenues	0	0	0	1,100
Operating Revenues	377,400	306,700	1,698,300	1,858,000
Natural gas	144,500	84,900	710,700	908,400
Operation and maintenance	95,000	103,200	306,500	310,200
Depreciation and amortization	58,100	50,900	169,200	149,000
Taxes, other than income taxes	43,000	32,100	150,300	124,000
Total Operating Expenses	340,600	271,100	1,336,700	1,491,600
Operating Income (Loss)	36,800	35,600	361,600	366,400
Net Economic Earnings (Loss)	4,200	12,300	240,600	248,400
Gas Marketing [Member] | Operating Segments [Member]
Revenues from external customers	64,100	15,200	171,400	73,300
Intersegment revenues	0	(100)	0	0
Operating Revenues	64,100	15,100	171,400	73,300
Natural gas	67,100	20,200	160,900	14,700
Operation and maintenance	3,200	3,200	9,100	13,600
Depreciation and amortization	300	300	1,000	900
Taxes, other than income taxes	400	200	800	900
Total Operating Expenses	71,000	23,900	171,800	30,100
Operating Income (Loss)	(6,900)	(8,800)	(400)	43,200
Net Economic Earnings (Loss)	400	(5,200)	15,300	37,900
Other Segments [Member] | Operating Segments [Member]
Revenues from external customers	6,500	5,900	14,600	15,100
Intersegment revenues	12,200	11,900	38,800	35,000
Operating Revenues	18,700	17,800	53,400	50,100
Natural gas	0	0	0	100
Operation and maintenance	8,000	9,200	28,000	28,900
Depreciation and amortization	2,000	1,900	6,000	5,500
Taxes, other than income taxes	700	300	2,200	1,700
Total Operating Expenses	10,700	11,400	36,200	36,200
Operating Income (Loss)	8,000	6,400	17,200	13,900
Net Economic Earnings (Loss)	$ (500)	$ (200)	$ (8,200)	$ (6,900)