COMMITMENTS AND CONTINGENT LIABILITIES - SCHEDULE OF FUTURE COMMITTED EXPENSES (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Current charges recoverable through BGSS	$ 98,600
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	534,913
2019	339,742
2020	178,178
2021	141,612
2022	123,149
Thereafter	668,441
Energy Services
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	385,277
2019	169,867
2020	59,424
2021	42,150
2022	25,486
Thereafter	22,057
Energy Services | Natural gas purchases
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	296,491
2019	114,817
2020	22,270
2021	11,488
2022	0
Thereafter	0
Energy Services | Storage demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	32,870
2019	22,638
2020	13,350
2021	9,041
2022	5,833
Thereafter	2,746
Energy Services | Pipeline demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	55,916
2019	32,412
2020	23,804
2021	21,621
2022	19,653
Thereafter	19,311
Natural Gas Distribution
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	149,636
2019	169,875
2020	118,754
2021	99,462
2022	97,663
Thereafter	646,384
Natural Gas Distribution | Natural gas purchases
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	51,050
2019	41,156
2020	2,514
2021	0
2022	0
Thereafter	0
Natural Gas Distribution | Storage demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	30,042
2019	26,628
2020	15,331
2021	8,231
2022	7,804
Thereafter	3,903
Natural Gas Distribution | Pipeline demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	68,544
2019	102,091
2020	100,909
2021	91,231
2022	89,859
Thereafter	$ 642,481
Minimum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, fixed period	1 year
Maximum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, fixed period	10 years