REVENUE - DISAGGREGATED REVENUE - TYPE (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
Revenues out of scope	362,906	587,618
Total operating revenues	615,036	811,767
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	167,961	147,539
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	62,919	56,045
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,589	18,934
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,661	1,631
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	605,322	802,129
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 | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,279	11,717
Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	213	228
Home Services and Other | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Home Services and Other | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Natural Gas Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	219,894	194,983
Revenues out of scope	(271)	4,982
Total operating revenues	219,623	199,965
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	153,222	133,690
Natural Gas Distribution | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	45,422	40,728
Natural Gas Distribution | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,589	18,934
Natural Gas Distribution | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,661	1,631
Clean Energy Ventures | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,018	7,141
Revenues out of scope	2,194	7,756
Total operating revenues	6,212	14,897
Clean Energy Ventures | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,460	2,132
Clean Energy Ventures | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,558	5,009
Clean Energy Ventures | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Clean Energy Ventures | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Energy Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,321	10,080
Revenues out of scope	360,983	574,880
Total operating revenues	368,304	584,960
Energy Services | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Energy Services | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,321	10,080
Energy Services | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Energy Services | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Midstream | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,405	0
Revenues out of scope	0	0
Total operating revenues	8,405	0
Midstream | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Midstream | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,405	0
Midstream | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Midstream | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0