REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									$ 782,331
Revenues out of scope									1,809,714
Total operating revenues	$ 479,081	$ 434,942	$ 866,255	$ 811,767	$ 647,326	$ 543,435	$ 1,019,043	$ 705,305	2,592,045	$ 2,915,109	$ 2,268,617
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									497,445
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									216,879
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									61,370
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									6,637
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues									2,551,683	$ 2,916,044	$ 2,222,712
Unregulated | Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									48,600
Revenues out of scope									0
Total operating revenues									48,600
Unregulated | Home Services and Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									47,655
Unregulated | Home Services and Other | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									945
Unregulated | Home Services and Other | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0
Unregulated | Home Services and Other | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0
Natural Gas Distribution | Regulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									680,151
Revenues out of scope									30,642
Total operating revenues									710,793
Natural Gas Distribution | Regulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									440,787
Natural Gas Distribution | Regulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									171,357
Natural Gas Distribution | Regulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									61,370
Natural Gas Distribution | Regulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									6,637
Clean Energy Ventures | Unregulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									22,121
Revenues out of scope									75,978
Total operating revenues									98,099
Clean Energy Ventures | Unregulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									9,003
Clean Energy Ventures | Unregulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									13,118
Clean Energy Ventures | Unregulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0
Clean Energy Ventures | Unregulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0
Energy Services | Unregulated | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									31,459
Revenues out of scope									1,703,094
Total operating revenues									1,734,553
Energy Services | Unregulated | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0
Energy Services | Unregulated | Operating Segments | Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									31,459
Energy Services | Unregulated | Operating Segments | Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0
Energy Services | Unregulated | Operating Segments | Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									$ 0