COMMITMENTS AND CONTINGENT LIABILITIES - SCHEDULE OF FUTURE COMMITTED EXPENSES (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Current charges recoverable through BGSS	$ 75,900
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	486,419
2019	334,822
2020	231,984
2021	182,870
2022	162,716
Thereafter	$ 782,866
Maximum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, contract term	10 years
Energy Services
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	$ 370,940
2019	190,624
2020	74,369
2021	44,054
2022	25,142
Thereafter	22,500
Energy Services | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	287,394
2019	117,129
2020	21,504
2021	11,443
2022	0
Thereafter	0
Energy Services | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	24,479
2019	22,933
2020	13,573
2021	9,263
2022	6,055
Thereafter	3,523
Energy Services | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	59,067
2019	50,562
2020	39,292
2021	23,348
2022	19,087
Thereafter	$ 18,977
Energy Services | Minimum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, contract term	1 year
Natural Gas Distribution
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	$ 115,479
2019	144,198
2020	157,615
2021	138,816
2022	137,574
Thereafter	760,366
Natural Gas Distribution | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	39,542
2019	41,287
2020	40,046
2021	38,234
2022	38,794
Thereafter	79,894
Natural Gas Distribution | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	22,531
2019	27,193
2020	16,482
2021	9,380
2022	8,952
Thereafter	11,373
Natural Gas Distribution | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	53,406
2019	75,718
2020	101,087
2021	91,202
2022	89,828
Thereafter	$ 669,099