EMPLOYEE BENEFIT PLANS - SUMMARY OF CHANGE IN FUNDED STATUS AND LIABILITIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Postemployment employee (liability)
Noncurrent	$ (128,888)	$ (141,604)
Pension
Change in Benefit Obligation
Benefit obligation at beginning of year	293,654	255,987
Service cost	8,347	7,591	$ 7,485
Interest cost	9,771	11,342	10,199
Plan participants’ contributions	45	47
Actuarial (gain) loss	(5,995)	26,369
Benefits paid, net of retiree subsidies received	(7,987)	(7,682)
Benefit obligation at end of year	297,835	293,654	255,987
Change in plan assets
Fair value of plan assets at beginning of year	249,875	199,123
Actual return on plan assets	29,736	28,316
Employer contributions	74	30,071
Benefits paid, net of plan participants’ contributions	(7,942)	(7,635)
Fair value of plan assets at end of year	271,743	249,875	199,123
Funded status	(26,092)	(43,779)
Postemployment employee (liability)
Current	(158)	(79)
Noncurrent	(25,934)	(43,700)
Total	(26,092)	(43,779)
OPEB
Change in Benefit Obligation
Benefit obligation at beginning of year	160,393	138,367
Service cost	4,380	4,521	4,253
Interest cost	5,545	6,256	5,739
Plan participants’ contributions	120	104
Actuarial (gain) loss	8,985	15,590
Benefits paid, net of retiree subsidies received	(4,333)	(4,445)
Benefit obligation at end of year	175,090	160,393	138,367
Change in plan assets
Fair value of plan assets at beginning of year	62,035	57,269
Actual return on plan assets	7,953	5,872
Employer contributions	6,049	3,235	5,700
Benefits paid, net of plan participants’ contributions	(4,503)	(4,341)
Fair value of plan assets at end of year	71,534	62,035	$ 57,269
Funded status	(103,556)	(98,358)
Postemployment employee (liability)
Current	(602)	(454)
Noncurrent	(102,954)	(97,904)
Total	$ (103,556)	$ (98,358)