EMPLOYEE BENEFIT PLANS, REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Pension [Member]
Amounts amortized to net periodic costs:
Net actuarial (loss) gain	$ (6,985)	$ (5,596)	$ (7,646)
Prior service (cost) credit	(111)	(111)	(108)
Net transition obligation	0	0	0
OPEB [Member]
Amounts amortized to net periodic costs:
Net actuarial (loss) gain	(2,943)	(2,500)	(3,857)
Prior service (cost) credit	364	357	355
Net transition obligation	0	(11)	(26)
Regulatory Assets [Member] | Pension [Member]
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	61,794	56,664
Amounts arising during the period, Net actuarial loss (gain)	30,579	10,563
Amounts amortized to net periodic costs:
Net actuarial (loss) gain	(5,305)	(5,326)
Prior service (cost) credit	(108)	(107)
Net transition obligation	0	0
Regulatory Assets, Balance at end of period	86,960	61,794	56,664
Regulatory Assets [Member] | OPEB [Member]
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	43,774	41,812
Amounts arising during the period, Net actuarial loss (gain)	9,563	4,277
Amounts amortized to net periodic costs:
Net actuarial (loss) gain	(2,911)	(2,607)
Prior service (cost) credit	311	303
Net transition obligation	0	(11)
Regulatory Assets, Balance at end of period	50,737	43,774	41,812
Accumulated Other Comprehensive Income (Loss) [Member] | Pension [Member]
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	17,581	14,427
Amounts arising during the period, Net actuarial loss (gain)	9,742	6,243
Amounts amortized to net periodic costs:
Net actuarial (loss) gain	(1,680)	(3,085)
Prior service (cost) credit	(3)	(4)
Net transition obligation	0	0
Accumulated Other Comprehensive Income (Loss), Balance at end of period	25,640	17,581	14,427
Accumulated Other Comprehensive Income (Loss) [Member] | OPEB [Member]
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	117	(2,142)
Amounts arising during the period, Net actuarial loss (gain)	1,103	2,098
Amounts amortized to net periodic costs:
Net actuarial (loss) gain	(32)	107
Prior service (cost) credit	54	54
Net transition obligation	0	0
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 1,242	$ 117	$ (2,142)