REVENUE - DISAGGREGATED REVENUE - PRODUCT (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
Alternative revenue programs	(2,665)	2,025	17,465	9,059
Derivative Instruments	144,418	296,082	825,261	1,434,104
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
Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	124,888	115,525	611,650	598,676
Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	15,399	3,876	52,502	26,204
Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,126	7,890	24,237	23,533
Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,243	5,193	13,404	14,373
Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,294	4,745	13,770	16,944
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	287,334	422,253	1,520,648	2,084,987
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	124,888	115,525	611,650	598,676
Alternative revenue programs	(2,665)	2,025	17,465	9,059
Derivative Instruments	6,309	3,232	16,260	14,432
Revenues out of scope	3,644	5,257	33,725	23,491
Total operating revenues	128,532	120,782	645,375	622,167
Operating Segments | Natural Gas Distribution | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	124,888	115,525	611,650	598,676
Operating Segments | Natural Gas Distribution | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Natural Gas Distribution | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Natural Gas Distribution | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Natural Gas Distribution | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,294	4,745	13,770	16,944
Alternative revenue programs	0	0	0	0
Derivative Instruments	8,102	6,705	11,833	20,763
Revenues out of scope	8,102	6,705	11,833	20,763
Total operating revenues	13,396	11,450	25,603	37,707
Operating Segments | Clean Energy Ventures | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Clean Energy Ventures | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Clean Energy Ventures | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Clean Energy Ventures | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Clean Energy Ventures | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,294	4,745	13,770	16,944
Operating Segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,536	3,876	20,491	26,204
Alternative revenue programs	0	0	0	0
Derivative Instruments	130,007	286,145	797,168	1,398,909
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
Operating Segments | Energy Services | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Energy Services | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,536	3,876	20,491	26,204
Operating Segments | Energy Services | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Energy Services | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Energy Services | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,143	0	29,949	0
Alternative revenue programs	0	0	0	0
Derivative Instruments	0	0	0	0
Revenues out of scope	0	0	0	0
Total operating revenues	11,143	0	29,949	0
Operating Segments | Midstream | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Midstream | Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,863	0	32,011	0
Operating Segments | Midstream | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Midstream | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Midstream | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
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 | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,163	12,627	36,694	36,253
Alternative revenue programs	0	0	0	0
Derivative Instruments	0	0	0	0
Revenues out of scope	0	0	0	0
Total operating revenues	12,163	12,627	36,694	36,253
Home Services and Other | Home Services and Other | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	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	0	0
Home Services and Other | Home Services and Other | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,126	7,890	24,237	23,533
Home Services and Other | Home Services and Other | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,243	5,193	13,404	14,373
Home Services and Other | Home Services and Other | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(926)	(456)	(3,009)	(1,653)
Revenues out of scope	197	62	(1,656)	(8,276)
Eliminations | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Eliminations | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Eliminations | Energy Services
Disaggregation of Revenue [Line Items]
Revenues out of scope	197	62	(1,656)	(8,276)
Eliminations | Midstream
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(720)	0	(2,062)	0
Revenues out of scope	0	0	0	0
Eliminations | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(206)	(456)	(947)	(1,653)
Revenues out of scope	$ 0	$ 0	$ 0	$ 0