EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Pension
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (7,537)	$ (8,827)	$ (7,281)
Prior service (cost) credit	(106)	(111)	(111)
OPEB
Amounts amortized to net periodic costs:
Net actuarial (loss)	(4,660)	(4,370)	(3,274)
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	78,605	94,941
Amounts arising during the period, Net actuarial loss (gain)	(6,090)	(9,429)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(6,177)	(6,799)
Prior service (cost) credit	(105)	(108)
Regulatory Assets, Balance at end of period	66,233	78,605	94,941
Regulatory Assets | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	60,460	59,147
Amounts arising during the period, Net actuarial loss (gain)	12,378	5,211
Amounts amortized to net periodic costs:
Net actuarial (loss)	(4,464)	(4,209)
Prior service (cost) credit	311	311
Regulatory Assets, Balance at end of period	68,685	60,460	59,147
Accumulated Other Comprehensive Income (Loss) | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	19,415	28,436
Amounts arising during the period, Net actuarial loss (gain)	(3,422)	(6,990)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(1,359)	(2,028)
Prior service (cost) credit	(1)	(3)
Accumulated Other Comprehensive Income (Loss), Balance at end of period	14,633	19,415	28,436
Accumulated Other Comprehensive Income (Loss) | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	4,967	4,486
Amounts arising during the period, Net actuarial loss (gain)	2,834	587
Amounts amortized to net periodic costs:
Net actuarial (loss)	(196)	(160)
Prior service (cost) credit	54	54
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 7,659	$ 4,967	$ 4,486