REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 225,727	$ 252,130
Revenues out of scope	228,578	362,906
Total operating revenues	454,305	615,036
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	152,150	167,961
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	50,666	62,919
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	21,104	19,589
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,807	1,661
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	444,680	605,322
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0
Total operating revenues	12,410	12,492
Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,204	12,279
Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	206	213
Home Services and Other | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Home Services and Other | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Natural Gas Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	189,364	219,894
Revenues out of scope	6,365	(271)
Total operating revenues	195,729	219,623
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	137,224	153,222
Natural Gas Distribution | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	29,229	45,422
Natural Gas Distribution | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	21,104	19,589
Natural Gas Distribution | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,807	1,661
Clean Energy Ventures | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,078	4,018
Revenues out of scope	1,292	2,194
Total operating revenues	6,370	6,212
Clean Energy Ventures | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,722	2,460
Clean Energy Ventures | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,356	1,558
Clean Energy Ventures | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Clean Energy Ventures | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Energy Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6,428	7,321
Revenues out of scope	220,921	360,983
Total operating revenues	227,349	368,304
Energy Services | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Energy Services | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6,428	7,321
Energy Services | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Energy Services | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Storage and Transportation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,447	8,405
Revenues out of scope	0	0
Total operating revenues	12,447	8,405
Storage and Transportation | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Storage and Transportation | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,447	8,405
Storage and Transportation | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Storage and Transportation | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0