EMPLOYEE BENEFIT PLANS, REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Pension
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (7,281)	$ (6,985)	$ (5,596)
Prior service (cost) credit	(111)	(111)	(111)
OPEB
Amounts amortized to net periodic costs:
Net actuarial (loss)	(3,274)	(2,943)	(2,500)
Prior service (cost) credit	365	364	357
Regulatory Assets | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	86,960	61,794
Amounts arising during the period, Net actuarial loss (gain)	13,696	30,579
Amounts amortized to net periodic costs:
Net actuarial (loss)	(5,607)	(5,305)
Prior service (cost) credit	(108)	(108)
Regulatory Assets, Balance at end of period	94,941	86,960	61,794
Regulatory Assets | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	50,737	43,774
Amounts arising during the period, Net actuarial loss (gain)	11,274	9,563
Amounts amortized to net periodic costs:
Net actuarial (loss)	(3,175)	(2,911)
Prior service (cost) credit	311	311
Regulatory Assets, Balance at end of period	59,147	50,737	43,774
Accumulated Other Comprehensive Income (Loss) | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	25,640	17,581
Amounts arising during the period, Net actuarial loss (gain)	4,475	9,742
Amounts amortized to net periodic costs:
Net actuarial (loss)	(1,676)	(1,680)
Prior service (cost) credit	(3)	(3)
Accumulated Other Comprehensive Income (Loss), Balance at end of period	28,436	25,640	17,581
Accumulated Other Comprehensive Income (Loss) | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	1,242	117
Amounts arising during the period, Net actuarial loss (gain)	3,289	1,103
Amounts amortized to net periodic costs:
Net actuarial (loss)	(99)	(32)
Prior service (cost) credit	54	54
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 4,486	$ 1,242	$ 117