COMMITMENTS AND CONTINGENT LIABILITIES - SCHEDULE OF FUTURE COMMITTED EXPENSES (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2018 USD ($)
Long-term Purchase Commitment [Line Items]
Current charges recoverable through BGSS	$ 54,500
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	406,118
2019	372,780
2020	250,023
2021	192,170
2022	170,206
Thereafter	$ 793,685
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	$ 311,549
2019	220,200
2020	80,894
2021	47,392
2022	27,092
Thereafter	26,084
Energy Services | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	256,179
2019	134,005
2020	20,506
2021	11,112
2022	0
Thereafter	0
Energy Services | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	19,299
2019	30,207
2020	17,581
2021	11,137
2022	6,053
Thereafter	3,521
Energy Services | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	36,071
2019	55,988
2020	42,807
2021	25,143
2022	21,039
Thereafter	$ 22,563
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	$ 94,569
2019	152,580
2020	169,129
2021	144,778
2022	143,114
Thereafter	767,601
Natural Gas Distribution | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	40,088
2019	41,715
2020	39,370
2021	37,655
2022	38,684
Thereafter	80,788
Natural Gas Distribution | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	15,479
2019	31,362
2020	24,245
2021	14,243
2022	12,922
Thereafter	15,070
Natural Gas Distribution | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2018	39,002
2019	79,503
2020	105,514
2021	92,880
2022	91,508
Thereafter	$ 671,743