REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 317,155			$ 541,304
Revenues out of scope	549,100			1,136,718
Total operating revenues	866,255		$ 1,019,043	1,678,022	$ 1,724,348
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	218,126			365,665
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	76,831			132,876
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20,776			39,710
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,422			3,053
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	860,605		$ 1,055,243	1,662,734	$ 1,757,007
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		$ 12,300		24,800
Unregulated | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,681			23,626
Revenues out of scope	0			0
Total operating revenues	11,681			23,626
Unregulated | Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,417			23,134
Unregulated | Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	264			492
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	288,168			483,151
Revenues out of scope	13,252			18,234
Total operating revenues	301,420			501,385
Natural Gas Distribution | Regulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	204,518			338,208
Natural Gas Distribution | Regulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	61,452			102,180
Natural Gas Distribution | Regulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20,776			39,710
Natural Gas Distribution | Regulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,422			3,053
Clean Energy Ventures | Unregulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,058			12,199
Revenues out of scope	6,302			14,058
Total operating revenues	11,360			26,257
Clean Energy Ventures | Unregulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,191			4,323
Clean Energy Ventures | Unregulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,867			7,876
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	12,248			22,328
Revenues out of scope	529,546			1,104,426
Total operating revenues	541,794			1,126,754
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	12,248			22,328
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