EMPLOYEE BENEFIT PLANS, SUMMARY OF CHANGE IN FUNDED STATUS AND LIABILITIES (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Dec. 31, 2015	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2014
Change in plan assets
Employer contributions		$ 30,000			$ 0
Postemployment employee (liability)
Noncurrent			$ (141,604)		(137,414)
Pension
Change in Benefit Obligation
Benefit obligation at beginning of year	[1]	255,987	255,987		227,699
Service cost			7,591	[1]	7,485	[1]	$ 6,143
Interest cost			11,342	[1]	10,199	[1]	10,066
Plan participants’ contributions	[1],[2]		47		47
Actuarial loss	[1]		26,369		17,418
Benefits paid, net of retiree subsidies received	[1]		(7,682)		(6,861)
Benefit obligation at end of year	[1]		293,654		255,987		227,699
Change in plan assets
Fair value of plan assets at beginning of year	[1]	199,123	199,123		211,653
Actual return on plan assets	[1]		28,316		(5,813)
Employer contributions	[1]		30,071		97
Benefits paid, net of plan participants’ contributions	[1],[2]		(7,635)		(6,814)
Fair value of plan assets at end of year	[1]		249,875		199,123		211,653
Funded status	[1]		(43,779)		(56,864)
Postemployment employee (liability)
Current	[1]		(79)		(71)
Noncurrent	[1]		(43,700)		(56,793)
Total	[1]		(43,779)		(56,864)
OPEB
Change in Benefit Obligation
Benefit obligation at beginning of year		138,367	138,367		127,773
Service cost			4,521		4,253		3,923
Interest cost			6,256		5,739		5,734
Plan participants’ contributions	[2]		104		60
Actuarial loss			15,590		3,891
Benefits paid, net of retiree subsidies received			(4,445)		(3,349)
Benefit obligation at end of year			160,393		138,367		127,773
Change in plan assets
Fair value of plan assets at beginning of year		$ 57,269	57,269		56,909
Actual return on plan assets			5,872		(1,799)
Employer contributions			3,235		5,672		5,000
Benefits paid, net of plan participants’ contributions	[2]		(4,341)		(3,513)
Fair value of plan assets at end of year			62,035		57,269		$ 56,909
Funded status			(98,358)		(81,098)
Postemployment employee (liability)
Current			(454)		(477)
Noncurrent			(97,904)		(80,621)
Total			$ (98,358)		$ (81,098)

[1] Includes the Company’s PEP.

[2] Prior to July 1, 1998, employees were eligible to elect an additional participant contribution to enhance their benefits and contributions made during the periods were insignificant.