REVENUE - DISAGGREGATED REVENUE - PRODUCT (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 252,130	$ 224,149
Alternative revenue programs	(1,943)	(867)
Derivative Instruments	366,960	590,792
Revenues out of scope	362,906	587,618
Total operating revenues	615,036	811,767
Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	219,894	194,983
Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	16,393	10,080
Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,038	7,796
Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,869	4,694
Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,018	7,141
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	605,322	802,129
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	219,894	194,983
Alternative revenue programs	(1,943)	(867)
Derivative Instruments	1,672	5,849
Revenues out of scope	(271)	4,982
Total operating revenues	219,623	199,965
Operating Segments | Natural Gas Distribution | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	219,894	194,983
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 | 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	4,018	7,141
Alternative revenue programs	0	0
Derivative Instruments	2,194	7,756
Revenues out of scope	2,194	7,756
Total operating revenues	6,212	14,897
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 | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,018	7,141
Operating Segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,321	10,080
Alternative revenue programs	0	0
Derivative Instruments	363,094	577,187
Revenues out of scope	360,983	574,880
Total operating revenues	368,304	584,960
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	7,321	10,080
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 | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,405	0
Alternative revenue programs	0	0
Derivative Instruments	0	0
Revenues out of scope	0	0
Total operating revenues	8,405	0
Operating Segments | Midstream | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Midstream | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,072	0
Operating Segments | Midstream | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Midstream | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Midstream | 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	12,492	11,945
Home Services and Other | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,492	11,945
Alternative revenue programs	0	0
Derivative Instruments	0	0
Revenues out of scope	0	0
Total operating revenues	12,492	11,945
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	8,038	7,796
Home Services and Other | Home Services and Other | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,869	4,694
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	(1,082)	(545)
Revenues out of scope	(2,111)	(2,307)
Eliminations | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0
Eliminations | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0
Eliminations | Energy Services
Disaggregation of Revenue [Line Items]
Revenues out of scope	(2,111)	(2,307)
Eliminations | Midstream
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(667)	0
Revenues out of scope	0	0
Eliminations | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(415)	(545)
Revenues out of scope	$ 0	$ 0