REVENUE - DISAGGREGATED REVENUE - TYPE (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
Revenues out of scope	298,169		1,434,887
Total operating revenues	434,942	$ 543,435	2,112,964	$ 2,267,783
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	78,676		444,341
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	44,067		176,943
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,296		52,006
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,734		4,787
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
Unregulated | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,627		36,253
Revenues out of scope	0		0
Total operating revenues	12,627		36,253
Unregulated | Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,388		35,522
Unregulated | Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	239		731
Unregulated | Home Services and Other | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Unregulated | Home Services and Other | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Natural Gas Distribution | Regulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	115,525		598,676
Revenues out of scope	5,257		23,491
Total operating revenues	120,782		622,167
Natural Gas Distribution | Regulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	63,984		402,192
Natural Gas Distribution | Regulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	37,511		139,691
Natural Gas Distribution | Regulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,296		52,006
Natural Gas Distribution | Regulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,734		4,787
Clean Energy Ventures | Unregulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,745		16,944
Revenues out of scope	6,705		20,763
Total operating revenues	11,450		37,707
Clean Energy Ventures | Unregulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,304		6,627
Clean Energy Ventures | Unregulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,441		10,317
Clean Energy Ventures | Unregulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Clean Energy Ventures | Unregulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Energy Services | Unregulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,876		26,204
Revenues out of scope	286,207		1,390,633
Total operating revenues	290,083		1,416,837
Energy Services | Unregulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Energy Services | Unregulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,876		26,204
Energy Services | Unregulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Energy Services | Unregulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0		$ 0