EMPLOYEE BENEFIT PLANS - SUMMARY OF CHANGE IN FUNDED STATUS AND LIABILITIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Postemployment employee (liability)
Noncurrent	$ (246,517)	$ (137,007)
Pension
Change in Benefit Obligation
Benefit obligation at beginning of year	298,575	297,835
Service cost	7,381	8,139	$ 8,347
Interest cost	12,173	10,493	9,771
Plan participants’ contributions	43	45
Special termination benefit	0	3,730	0
Actuarial loss (gain)	52,549	(12,846)
Benefits paid, net of retiree subsidies received	(10,244)	(8,821)
Benefit obligation at end of year	360,477	298,575	297,835
Change in plan assets
Fair value of plan assets at beginning of year	279,410	271,743
Actual return on plan assets	19,194	16,306
Employer contributions	231	137
Benefits paid, net of plan participants’ contributions	(10,201)	(8,776)
Fair value of plan assets at end of year	288,634	279,410	271,743
Funded status	(71,843)	(19,165)
Postemployment employee (liability)
Current	(603)	(294)
Noncurrent	(71,240)	(18,871)
Total	(71,843)	(19,165)
OPEB
Change in Benefit Obligation
Benefit obligation at beginning of year	196,785	175,090
Service cost	4,404	4,607	4,380
Interest cost	8,324	6,365	5,545
Plan participants’ contributions	210	161
Special termination benefit	0	490	0
Actuarial loss (gain)	54,700	15,145
Benefits paid, net of retiree subsidies received	(4,420)	(5,073)
Benefit obligation at end of year	260,003	196,785	175,090
Change in plan assets
Fair value of plan assets at beginning of year	77,980	71,534
Actual return on plan assets	2,499	5,284
Employer contributions	7,926	6,222	6,000
Benefits paid, net of plan participants’ contributions	(4,479)	(5,060)
Fair value of plan assets at end of year	83,926	77,980	$ 71,534
Funded status	(176,077)	(118,805)
Postemployment employee (liability)
Current	(800)	(669)
Noncurrent	(175,277)	(118,136)
Total	$ (176,077)	$ (118,805)