REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Utility	$ 310,167	$ 297,220	$ 505,896	$ 516,843
Nonutility	492,020	342,394	750,596	737,807
Total operating revenues	802,187	639,614	1,256,492	1,254,650
Depreciation and amortization	26,848	27,516	54,210	52,153
Interest income	1,057	1,721	1,726	1,907
Interest expense, net of capitalized interest	20,153	19,203	39,939	35,273
Income tax provision (benefit)	39,057	16,284	56,498	32,755
Equity in earnings of affiliates	2,804	3,589	5,479	6,978
Net financial earnings (loss)	170,604	84,291	215,261	119,222
Capital expenditures	136,007	101,166	245,887	227,349
Investments in equity investees	196	757	482	1,266
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues	793,138	627,992	1,237,818	1,233,314
Depreciation and amortization	26,537	27,127	53,821	51,970
Interest income	1,167	2,273	1,943	3,114
Interest expense, net of capitalized interest	18,425	18,866	37,095	35,238
Income tax provision (benefit)	36,909	14,224	54,958	30,479
Net financial earnings (loss)	172,908	84,252	217,109	117,811
Capital expenditures	135,882	101,277	245,076	227,334
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility	310,167	297,220	505,896	516,843
Total operating revenues	310,167	297,220	505,896	516,843
Depreciation and amortization	19,475	18,100	38,644	34,917
Interest income	76	136	158	243
Interest expense, net of capitalized interest	9,006	7,473	17,980	15,305
Income tax provision (benefit)	15,622	19,702	23,989	28,337
Net financial earnings (loss)	80,541	86,336	130,008	130,192
Capital expenditures	97,239	72,042	176,485	149,839
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility	6,476	5,995	12,846	12,207
Total operating revenues	6,476	5,995	12,846	12,207
Depreciation and amortization	4,685	6,603	10,118	12,919
Interest income	240	0	240	0
Interest expense, net of capitalized interest	5,266	4,835	10,300	9,327
Income tax provision (benefit)	(2,714)	(4,134)	(5,800)	(7,968)
Net financial earnings (loss)	(8,872)	(8,829)	(19,146)	(17,008)
Capital expenditures	18,549	24,087	40,884	69,786
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	459,766	313,959	687,115	682,263
Total operating revenues	459,766	313,959	687,115	682,263
Depreciation and amortization	13	27	55	56
Interest income	0	52	0	89
Interest expense, net of capitalized interest	575	937	1,255	2,163
Income tax provision (benefit)	23,128	(2,449)	35,250	8,303
Net financial earnings (loss)	96,528	2,487	98,028	(2,635)
Operating Segments | Storage and Transportation
Segment Reporting Information [Line Items]
Nonutility	13,257	10,401	25,704	18,806
Total operating revenues	13,257	10,401	25,704	18,806
Depreciation and amortization	2,364	2,397	5,004	4,078
Interest income	851	2,085	1,545	2,782
Interest expense, net of capitalized interest	3,578	5,621	7,560	8,443
Income tax provision (benefit)	873	1,105	1,519	1,807
Equity in earnings of affiliates	3,386	3,921	6,579	7,585
Net financial earnings (loss)	4,711	4,258	8,219	7,262
Capital expenditures	20,094	5,148	27,707	7,709
Investments in equity investees	196	757	482	1,266
Intercompany
Segment Reporting Information [Line Items]
Nonutility	252	326	419	741
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility	2,803	(258)	4,931	1,853
Intercompany | Storage and Transportation
Segment Reporting Information [Line Items]
Nonutility	669	675	1,326	1,342
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility	12,521	12,039	24,931	24,531
Total operating revenues	12,521	12,039	24,931	24,531
Depreciation and amortization	227	250	487	496
Interest income	127	208	259	699
Interest expense, net of capitalized interest	1,728	565	2,844	837
Income tax provision (benefit)	(59)	1,956	59	2,175
Net financial earnings (loss)	747	148	685	1,257
Capital expenditures	125	(111)	811	15
Eliminations
Segment Reporting Information [Line Items]
Nonutility	(3,724)	(743)	(6,676)	(3,936)
Depreciation and amortization	84	139	(98)	(313)
Interest income	(237)	(760)	(476)	(1,906)
Interest expense, net of capitalized interest	0	(228)	0	(802)
Income tax provision (benefit)	2,207	104	1,481	101
Equity in earnings of affiliates	(582)	(332)	(1,100)	(607)
Net financial earnings (loss)	$ (3,051)	$ (109)	$ (2,533)	$ 154