EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - OPEB - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (930)	$ (1,616)	$ (3,721)	$ (3,233)
Prior service credit	50	$ 91	99	182
Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period			111,629	68,685	$ 68,685
Amounts arising during the period, Net actuarial loss (gain)			(40,074)		48,452
Amounts amortized to net periodic costs:
Net actuarial (loss)			(2,441)		(5,820)
Prior service credit			45		312
Regulatory Assets, Balance at end of period	69,159		69,159		111,629
Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period			16,328	$ 7,659	7,659
Amounts arising during the period, Net actuarial loss (gain)			(7,059)		9,264
Amounts amortized to net periodic costs:
Net actuarial (loss)			(350)		(648)
Prior service credit			4		53
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 8,923		$ 8,923		$ 16,328