REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 157,024	$ 136,773	$ 712,554	$ 678,077
Revenues out of scope	141,950	298,169	841,070	1,434,887
Total operating revenues	298,974	434,942	1,553,624	2,112,964
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	103,527	78,676	485,564	444,341
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	37,754	44,067	164,540	176,943
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	14,331	12,296	58,043	52,006
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,412	1,734	4,407	4,787
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	287,334	422,253	1,520,648	2,084,987
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Total operating revenues	12,163	12,627	36,694	36,253
Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,845	12,388	35,955	35,522
Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	318	239	739	731
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	124,888	115,525	611,650	598,676
Revenues out of scope	3,644	5,257	33,725	23,491
Total operating revenues	128,532	120,782	645,375	622,167
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	89,095	63,984	442,093	402,192
Natural Gas Distribution | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20,050	37,511	107,107	139,691
Natural Gas Distribution | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	14,331	12,296	58,043	52,006
Natural Gas Distribution | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,412	1,734	4,407	4,787
Clean Energy Ventures | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,294	4,745	13,770	16,944
Revenues out of scope	8,102	6,705	11,833	20,763
Total operating revenues	13,396	11,450	25,603	37,707
Clean Energy Ventures | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,587	2,304	7,516	6,627
Clean Energy Ventures | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,707	2,441	6,254	10,317
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	3,536	3,876	20,491	26,204
Revenues out of scope	130,204	286,207	795,512	1,390,633
Total operating revenues	133,740	290,083	816,003	1,416,837
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	3,536	3,876	20,491	26,204
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
Midstream | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,143	0	29,949	0
Revenues out of scope	0	0	0	0
Total operating revenues	11,143	0	29,949	0
Midstream | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Midstream | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,143	0	29,949	0
Midstream | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Midstream | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0	$ 0	$ 0