REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 339,376	$ 303,400	$ 565,103	$ 555,530
Revenues out of scope	462,811	336,214	691,389	699,120
Total operating revenues	802,187	639,614	1,256,492	1,254,650
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	240,100	214,076	392,250	382,037
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	70,386	63,867	121,052	126,786
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	28,190	24,123	49,294	43,712
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	700	1,334	2,507	2,995
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	793,138	627,992	1,237,818	1,233,314
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Total operating revenues	12,521	12,039	24,931	24,531
Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,234	11,831	24,438	24,110
Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	287	208	493	421
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	296,888	266,868	486,252	486,762
Revenues out of scope	13,279	30,352	19,644	30,081
Total operating revenues	310,167	297,220	505,896	516,843
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	225,174	199,776	362,398	352,998
Natural Gas Distribution | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	42,824	41,635	72,053	87,057
Natural Gas Distribution | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	28,190	24,123	49,294	43,712
Natural Gas Distribution | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	700	1,334	2,507	2,995
Clean Energy Ventures | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,699	4,458	10,777	8,476
Revenues out of scope	777	1,537	2,069	3,731
Total operating revenues	6,476	5,995	12,846	12,207
Clean Energy Ventures | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,692	2,469	5,414	4,929
Clean Energy Ventures | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,007	1,989	5,363	3,547
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 | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,011	9,634	17,439	16,955
Revenues out of scope	448,755	304,325	669,676	665,308
Total operating revenues	459,766	313,959	687,115	682,263
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	11,011	9,634	17,439	16,955
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	13,257	10,401	25,704	18,806
Revenues out of scope	0	0	0	0
Total operating revenues	13,257	10,401	25,704	18,806
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	13,257	10,401	25,704	18,806
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