REVENUE - DISAGGREGATED REVENUE - PRODUCT (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		$ 136,773		$ 678,077
Alternative revenue programs		2,025		9,059
Derivative Instruments		296,082		1,434,104
Revenues out of scope		298,169		1,434,887
Total operating revenues		434,942	$ 543,435	2,112,964	$ 2,267,783
Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		115,525		598,676
Wholesale natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		3,876		26,204
Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		7,890		23,533
Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		5,193		14,373
Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		4,745		16,944
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		422,253	529,291	2,084,987	$ 2,286,298
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers			$ 13,100	37,900
Eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	[1]	(456)		(1,653)
Revenues out of scope	[1]	62		(8,276)
Regulated | Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		115,525		598,676
Alternative revenue programs		2,025		9,059
Derivative Instruments		3,232		14,432
Revenues out of scope		5,257		23,491
Total operating revenues		120,782		622,167
Regulated | Operating Segments | Natural gas utility sales | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		115,525		598,676
Regulated | Operating Segments | Wholesale natural gas services | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Regulated | Operating Segments | Service contracts | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Regulated | Operating Segments | Installations and maintenance | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Regulated | Operating Segments | Electricity sales | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Regulated | Eliminations | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues out of scope	[1]	0		0
Unregulated | Operating Segments | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		4,745		16,944
Alternative revenue programs		0		0
Derivative Instruments		6,705		20,763
Revenues out of scope		6,705		20,763
Total operating revenues		11,450		37,707
Unregulated | Operating Segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		3,876		26,204
Alternative revenue programs		0		0
Derivative Instruments		286,145		1,398,909
Revenues out of scope		286,207		1,390,633
Total operating revenues		290,083		1,416,837
Unregulated | Operating Segments | Natural gas utility sales | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Natural gas utility sales | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Wholesale natural gas services | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Wholesale natural gas services | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		3,876		26,204
Unregulated | Operating Segments | Service contracts | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Service contracts | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Installations and maintenance | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Installations and maintenance | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Operating Segments | Electricity sales | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		4,745		16,944
Unregulated | Operating Segments | Electricity sales | Energy Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		12,627		36,253
Alternative revenue programs		0		0
Derivative Instruments		0		0
Revenues out of scope		0		0
Total operating revenues		12,627		36,253
Unregulated | Home Services and Other | Natural gas utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Home Services and Other | Wholesale natural gas services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Home Services and Other | Service contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		7,890		23,533
Unregulated | Home Services and Other | Installations and maintenance
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		5,193		14,373
Unregulated | Home Services and Other | Electricity sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Unregulated | Eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	[1]	(456)		(1,653)
Revenues out of scope	[1]	0		0
Unregulated | Eliminations | Clean Energy Ventures
Disaggregation of Revenue [Line Items]
Revenues out of scope	[1]	0		0
Unregulated | Eliminations | Energy Services
Disaggregation of Revenue [Line Items]
Revenues out of scope	[1]	$ 62		$ (8,276)

[1] Consists of transactions between subsidiaries that are eliminated in consolidation.