COMMITMENTS AND CONTINGENT LIABILITIES - SCHEDULE OF FUTURE COMMITTED EXPENSES (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Current charges recoverable through BGSS	$ 131,200
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	511,685
2020	273,281
2021	198,498
2022	171,256
2023	119,900
Thereafter	583,146
Energy Services
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	328,718
2020	105,401
2021	54,067
2022	35,607
2023	10,322
Thereafter	2,089
Energy Services | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	226,513
2020	26,521
2021	0
2022	0
2023	0
Thereafter	0
Energy Services | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	28,237
2020	17,298
2021	11,716
2022	8,735
2023	2,280
Thereafter	1,032
Energy Services | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	73,968
2020	61,582
2021	42,351
2022	26,872
2023	8,042
Thereafter	$ 1,057
Energy Services | Minimum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, contract term	1 year
Energy Services | Maximum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, contract term	10 years
Natural Gas Distribution
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	$ 182,967
2020	167,880
2021	144,431
2022	135,649
2023	109,578
Thereafter	581,057
Natural Gas Distribution | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	51,763
2020	33,269
2021	33,607
2022	34,142
2023	36,155
Thereafter	8,889
Natural Gas Distribution | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	35,988
2020	27,025
2021	16,383
2022	11,680
2023	3,112
Thereafter	4,441
Natural Gas Distribution | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2019	95,216
2020	107,586
2021	94,441
2022	89,827
2023	70,311
Thereafter	$ 567,727