EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (5,765)	$ (7,537)	$ (8,827)
Prior service (cost) credit	(102)	(106)	(111)
OPEB
Amounts amortized to net periodic costs:
Net actuarial (loss)	(6,466)	(4,660)	(4,370)
Prior service (cost) credit	365	365	365
Regulatory Assets | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	66,233	78,605
Amounts arising during the period, Net actuarial loss (gain)	38,137	(6,090)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(4,662)	(6,177)
Prior service (cost) credit	(102)	(105)
Regulatory Assets, Balance at end of period	99,606	66,233	78,605
Regulatory Assets | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	68,685	60,460
Amounts arising during the period, Net actuarial loss (gain)	48,452	12,378
Amounts amortized to net periodic costs:
Net actuarial (loss)	(5,820)	(4,464)
Prior service (cost) credit	312	311
Regulatory Assets, Balance at end of period	111,629	68,685	60,460
Accumulated Other Comprehensive Income (Loss) | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	14,633	19,415
Amounts arising during the period, Net actuarial loss (gain)	14,271	(3,422)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(1,103)	(1,359)
Prior service (cost) credit	0	(1)
Accumulated Other Comprehensive Income (Loss), Balance at end of period	27,801	14,633	19,415
Accumulated Other Comprehensive Income (Loss) | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	7,659	4,967
Amounts arising during the period, Net actuarial loss (gain)	9,264	2,834
Amounts amortized to net periodic costs:
Net actuarial (loss)	(648)	(196)
Prior service (cost) credit	53	54
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 16,328	$ 7,659	$ 4,967