REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 432,643	$ 339,376	$ 716,733	$ 565,103
Revenues out of scope	479,673	462,811	871,425	691,389
Total operating revenues	912,316	802,187	1,588,158	1,256,492
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	302,301	240,100	470,356	392,250
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	95,130	70,386	186,198	121,052
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	34,636	28,190	57,311	49,294
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	576	700	2,868	2,507
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	901,626	793,138	1,567,968	1,237,818
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Total operating revenues	13,141	12,521	26,993	24,931
Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,105	12,234	26,880	24,438
Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	36	287	113	493
Home Services and Other | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Home Services and Other | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Natural Gas Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	381,523	296,888	604,982	486,252
Revenues out of scope	81,951	13,279	132,927	19,644
Total operating revenues	463,474	310,167	737,909	505,896
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	286,205	225,174	437,513	362,398
Natural Gas Distribution | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	60,106	42,824	107,290	72,053
Natural Gas Distribution | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	34,636	28,190	57,311	49,294
Natural Gas Distribution | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	576	700	2,868	2,507
Clean Energy Ventures | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,865	5,699	15,181	10,777
Revenues out of scope	3,962	777	6,829	2,069
Total operating revenues	11,827	6,476	22,010	12,846
Clean Energy Ventures | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,991	2,692	5,963	5,414
Clean Energy Ventures | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,874	3,007	9,218	5,363
Clean Energy Ventures | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Clean Energy Ventures | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Energy Services
Disaggregation of Revenue [Line Items]
Total operating revenues	10,300		32,400
Energy Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,309	11,011	45,188	17,439
Revenues out of scope	393,760	448,755	731,669	669,676
Total operating revenues	411,069	459,766	776,857	687,115
Energy Services | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Energy Services | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,309	11,011	45,188	17,439
Energy Services | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Energy Services | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Storage and Transportation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,805	13,257	24,389	25,704
Revenues out of scope	0	0	0	0
Total operating revenues	12,805	13,257	24,389	25,704
Storage and Transportation | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Storage and Transportation | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,805	13,257	24,389	25,704
Storage and Transportation | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Storage and Transportation | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0	$ 0	$ 0