REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 303,400	$ 317,155	$ 555,530	$ 541,304
Revenues out of scope	336,214	549,100	699,120	1,136,718
Total operating revenues	639,614	866,255	1,254,650	1,678,022
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	214,076	218,126	382,037	365,665
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	63,867	76,831	126,786	132,876
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	24,123	20,776	43,712	39,710
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,334	1,422	2,995	3,053
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	627,992	860,605	1,233,314	1,662,734
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Total operating revenues	12,039	11,681	24,531	23,626
Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,831	11,417	24,110	23,134
Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	208	264	421	492
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	266,868	288,168	486,762	483,151
Revenues out of scope	30,352	13,252	30,081	18,234
Total operating revenues	297,220	301,420	516,843	501,385
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	199,776	204,518	352,998	338,208
Natural Gas Distribution | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	41,635	61,452	87,057	102,180
Natural Gas Distribution | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	24,123	20,776	43,712	39,710
Natural Gas Distribution | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,334	1,422	2,995	3,053
Clean Energy Ventures | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,458	5,058	8,476	12,199
Revenues out of scope	1,537	6,302	3,731	14,058
Total operating revenues	5,995	11,360	12,207	26,257
Clean Energy Ventures | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,469	2,191	4,929	4,323
Clean Energy Ventures | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,989	2,867	3,547	7,876
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	9,634	12,248	16,955	22,328
Revenues out of scope	304,325	529,546	665,308	1,104,426
Total operating revenues	313,959	541,794	682,263	1,126,754
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	9,634	12,248	16,955	22,328
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	10,401	0	18,806	0
Revenues out of scope	0	0	0	0
Total operating revenues	10,401	0	18,806	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	10,401	0	18,806	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