EMPLOYEE BENEFIT PLANS - SUMMARY OF CHANGE IN FUNDED STATUS AND LIABILITIES (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Change in plan assets
Employer contributions			$ 0	$ 0
Postemployment employee (liability)
Noncurrent	$ (195,441,000)		(195,441,000)		$ (246,517,000)
Pension
Change in Benefit Obligation
Service cost	2,055,000	$ 1,846,000	4,111,000	3,691,000
Interest cost	2,646,000	3,043,000	5,293,000	6,086,000
OPEB
Change in Benefit Obligation
Benefit obligation at beginning of period			260,003,000	196,785,000	196,785,000
Service cost	1,072,000	1,101,000	2,427,000	2,202,000	4,404,000
Interest cost	1,509,000	$ 2,081,000	3,513,000	4,162,000	8,324,000
Plan participants’ contributions					210,000
Actuarial loss (gain)					54,700,000
Benefits paid, net of retiree subsidies received					(4,420,000)
Benefit obligation at end of period					260,003,000
Change in plan assets
Fair value of plan assets at beginning of period			83,926,000	$ 77,980,000	77,980,000
Actual return on plan assets					2,499,000
Employer contributions					7,926,000
Benefits paid, net of plan participants’ contributions					(4,479,000)
Fair value of plan assets at end of period					83,926,000
Funded status					(176,077,000)
Postemployment employee (liability)
Current	(800,000)		(800,000)		(800,000)
Noncurrent	(127,837,000)		(127,837,000)		(175,277,000)
Total	(128,637,000)		(128,637,000)		(176,077,000)
Affordable Care Act, Cadillac Tax Repeal | OPEB
Change in Benefit Obligation
Benefit obligation at beginning of period			260,003,000
Service cost			1,354,000
Interest cost			2,004,000
Plan participants’ contributions			58,000
Actuarial loss (gain)			(44,017,000)
Benefits paid, net of retiree subsidies received			(2,342,000)
Benefit obligation at end of period	217,060,000		217,060,000		260,003,000
Change in plan assets
Fair value of plan assets at beginning of period			83,925,000
Actual return on plan assets			4,676,000
Employer contributions			2,106,000
Benefits paid, net of plan participants’ contributions			(2,284,000)
Fair value of plan assets at end of period	88,423,000		88,423,000		$ 83,925,000
Funded status	$ (128,637,000)		$ (128,637,000)