REVENUE - DISAGGREGATED REVENUE - PRODUCT (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
Alternative revenue programs	22,073	7,901	20,130	7,034
Derivative Instruments	313,883	547,230	680,843	1,138,022
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
Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	266,868	288,168	486,762	483,151
Natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20,710	12,248	37,103	22,328
Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,073	7,847	16,111	15,643
Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,292	4,486	9,161	9,180
Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,458	5,058	8,476	12,199
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	627,992	860,605	1,233,314	1,662,734
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	266,868	288,168	486,762	483,151
Alternative revenue programs	22,073	7,901	20,130	7,034
Derivative Instruments	8,279	5,351	9,951	11,200
Revenues out of scope	30,352	13,252	30,081	18,234
Total operating revenues	297,220	301,420	516,843	501,385
Operating Segments | Natural Gas Distribution | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	266,868	288,168	486,762	483,151
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	4,458	5,058	8,476	12,199
Alternative revenue programs	0	0	0	0
Derivative Instruments	1,537	6,302	3,731	14,058
Revenues out of scope	1,537	6,302	3,731	14,058
Total operating revenues	5,995	11,360	12,207	26,257
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	4,458	5,058	8,476	12,199
Operating Segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,634	12,248	16,955	22,328
Alternative revenue programs	0	0	0	0
Derivative Instruments	304,067	535,577	667,161	1,112,764
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
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	9,634	12,248	16,955	22,328
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	10,401	0	18,806	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	10,401	0	18,806	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	11,076	0	20,148	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	12,039	11,681	24,531	23,626
Home Services and Other | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,039	11,681	24,531	23,626
Alternative revenue programs	0	0	0	0
Derivative Instruments	0	0	0	0
Revenues out of scope	0	0	0	0
Total operating revenues	12,039	11,681	24,531	23,626
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	8,073	7,847	16,111	15,643
Home Services and Other | Home Services and Other | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,292	4,486	9,161	9,180
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	(1,001)	(652)	(2,083)	(1,197)
Revenues out of scope	258	(6,031)	(1,853)	(8,338)
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	258	(6,031)	(1,853)	(8,338)
Eliminations | Midstream
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(675)	0	(1,342)	0
Revenues out of scope	0	0	0	0
Eliminations | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(326)	(652)	(741)	(1,197)
Revenues out of scope	$ 0	$ 0	$ 0	$ 0