REVENUE - DISAGGREGATED REVENUE - PRODUCT (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
Alternative revenue programs	1,568	(1,943)
Derivative instruments	229,138	366,960
Revenues out of scope	228,578	362,906
Total operating revenues	454,305	615,036
Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	189,364	219,894
Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,532	16,393
Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,259	8,038
Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,318	4,869
Renewable energy certificates
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	690
Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,388	4,018
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	444,680	605,322
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	189,364	219,894
Alternative revenue programs	1,568	(1,943)
Derivative instruments	4,797	1,672
Revenues out of scope	6,365	(271)
Total operating revenues	195,729	219,623
Operating Segments | Natural Gas Distribution | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	189,364	219,894
Operating Segments | Natural Gas Distribution | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Natural Gas Distribution | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Natural Gas Distribution | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Natural Gas Distribution | Renewable energy certificates
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Operating Segments | Natural Gas Distribution | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,078	4,018
Alternative revenue programs	0	0
Derivative instruments	1,292	2,194
Revenues out of scope	1,292	2,194
Total operating revenues	6,370	6,212
Operating Segments | Clean Energy Ventures | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Clean Energy Ventures | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Clean Energy Ventures | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Clean Energy Ventures | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Clean Energy Ventures | Renewable energy certificates
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	690
Operating Segments | Clean Energy Ventures | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,388	4,018
Operating Segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6,428	7,321
Alternative revenue programs	0	0
Derivative instruments	223,049	363,094
Revenues out of scope	220,921	360,983
Total operating revenues	227,349	368,304
Operating Segments | Energy Services | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Energy Services | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6,428	7,321
Operating Segments | Energy Services | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Energy Services | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Energy Services | Renewable energy certificates
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Operating Segments | Energy Services | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Storage and Transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,447	8,405
Alternative revenue programs	0	0
Derivative instruments	0	0
Revenues out of scope	0	0
Total operating revenues	12,447	8,405
Operating Segments | Storage and Transportation | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Storage and Transportation | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,104	9,072
Operating Segments | Storage and Transportation | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Storage and Transportation | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Storage and Transportation | Renewable energy certificates
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Operating Segments | Storage and Transportation | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
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 | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,410	12,492
Alternative revenue programs	0	0
Derivative instruments	0	0
Revenues out of scope	0	0
Total operating revenues	12,410	12,492
Home Services and Other | Home Services and Other | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Home Services and Other | Home Services and Other | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Home Services and Other | Home Services and Other | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,259	8,038
Home Services and Other | Home Services and Other | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,318	4,869
Home Services and Other | Home Services and Other | Renewable energy certificates
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Home Services and Other | Home Services and Other | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(824)	(1,082)
Revenues out of scope	(2,128)	(2,111)
Eliminations | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Revenues out of scope	0	0
Eliminations | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Revenues out of scope	0	0
Eliminations | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Revenues out of scope	(2,128)	(2,111)
Eliminations | Storage and Transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(657)	(667)
Revenues out of scope	0	0
Eliminations | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(167)	(415)
Revenues out of scope	$ 0	$ 0