REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Operating revenues	$ 657,913	$ 644,027	$ 1,125,123	$ 1,367,594
Depreciation and amortization	40,075	38,090	80,362	74,773
Interest income	2,036	2,236	4,062	4,195
Interest expense, net of capitalized interest	31,621	30,261	63,094	59,752
Income tax provision (benefit)	33,947	30,586	56,883	63,564
Equity in earnings of affiliates	738	1,131	2,398	2,077
Net financial earnings (loss)	138,576	112,310	211,020	222,594
Capital expenditures	122,680	117,188	237,302	263,111
ES
Segment Reporting Information [Line Items]
Operating revenues	9,500	9,500	19,000	29,500
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	643,161	630,762	1,095,537	1,340,076
Depreciation and amortization	40,669	37,866	80,727	74,331
Interest income	3,178	2,440	6,254	4,523
Interest expense, net of capitalized interest	31,413	30,072	62,670	59,441
Income tax provision (benefit)	28,790	27,431	51,120	61,984
Equity in earnings of affiliates	85	977	1,078	1,886
Net financial earnings (loss)	141,104	114,893	214,541	224,751
Capital expenditures	122,550	116,428	235,816	262,211
Operating Segments | NJNG
Segment Reporting Information [Line Items]
Operating revenues	462,863	400,500	755,956	757,909
Depreciation and amortization	27,464	25,319	54,381	50,209
Interest income	517	376	1,095	789
Interest expense, net of capitalized interest	14,850	13,879	29,601	27,588
Income tax provision (benefit)	24,476	23,924	35,132	38,307
Equity in earnings of affiliates	0	0	0	0
Net financial earnings (loss)	107,095	100,697	158,539	155,361
Capital expenditures	97,710	92,600	177,425	174,671
Operating Segments | CEV
Segment Reporting Information [Line Items]
Operating revenues	9,325	14,406	44,620	27,198
Depreciation and amortization	6,931	6,465	13,853	12,041
Interest income	0	0	0	0
Interest expense, net of capitalized interest	7,182	7,316	14,629	13,211
Income tax provision (benefit)	(1,594)	(3,005)	1,537	(4,842)
Equity in earnings of affiliates	0	0	0	0
Net financial earnings (loss)	(5,616)	(9,379)	4,906	(12,961)
Capital expenditures	11,317	13,706	37,083	57,699
Operating Segments | ES
Segment Reporting Information [Line Items]
Operating revenues	148,604	196,553	249,405	509,952
Depreciation and amortization	56	62	113	119
Interest income	143	380	271	649
Interest expense, net of capitalized interest	3,513	2,749	6,639	5,807
Income tax provision (benefit)	5,289	5,916	12,800	25,980
Equity in earnings of affiliates	0	0	0	0
Net financial earnings (loss)	37,644	21,125	45,475	73,658
Capital expenditures	0	0	0	0
Operating Segments | S&T
Segment Reporting Information [Line Items]
Operating revenues	22,369	19,303	45,556	45,017
Depreciation and amortization	6,218	6,020	12,380	11,962
Interest income	2,518	1,684	4,888	3,085
Interest expense, net of capitalized interest	5,868	6,128	11,801	12,835
Income tax provision (benefit)	619	596	1,651	2,539
Equity in earnings of affiliates	85	977	1,078	1,886
Net financial earnings (loss)	1,981	2,450	5,621	8,693
Capital expenditures	13,523	10,122	21,308	29,841
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(2,731)	2,099	(2,852)	11,943
Intersegment Eliminations | NJNG
Segment Reporting Information [Line Items]
Operating revenues	338	338	675	675
Intersegment Eliminations | CEV
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Intersegment Eliminations | ES
Segment Reporting Information [Line Items]
Operating revenues	(3,742)	177	(4,875)	8,560
Intersegment Eliminations | S&T
Segment Reporting Information [Line Items]
Operating revenues	673	1,584	1,348	2,708
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Operating revenues	14,752	13,265	29,586	27,518
Depreciation and amortization	271	224	500	442
Interest income	332	734	688	1,388
Interest expense, net of capitalized interest	208	189	424	311
Income tax provision (benefit)	520	488	468	705
Equity in earnings of affiliates	0	0	0	0
Net financial earnings (loss)	384	813	(216)	784
Capital expenditures	130	760	1,486	900
Corporate Reconciling Items And Eliminations
Segment Reporting Information [Line Items]
Operating revenues	153	183	153	196
Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(2,282)		(12,139)
Depreciation and amortization	(865)	0	(865)	0
Interest income	(1,474)	(938)	(2,880)	(1,716)
Interest expense, net of capitalized interest	0	0	0	0
Income tax provision (benefit)	4,637	2,667	5,295	875
Equity in earnings of affiliates	653	154	1,320	191
Net financial earnings (loss)	(2,912)	(3,396)	(3,305)	(2,941)
Capital expenditures	0	$ 0	0	$ 0
Eliminations And Reconciling Items
Segment Reporting Information [Line Items]
Operating revenues	$ 2,578		$ 2,699