SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PENSION AND POSTEMPLOYMENT PLANS (Details)		3 Months Ended	12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2018 USD ($) plan	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of noncontributory defined benefit retirement plans (in plans) | plan			2
Number of noncontributory medical and life insurance plans (in plans) | plan			2
Pension
Defined Benefit Plan Disclosure [Line Items]
Employer discretionary contributions		$ 30,000,000	$ 0.0	$ 0
Employer contributions	[1]		137,000	74,000
OPEB
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			$ 6,222,000	$ 6,049,000	$ 3,200,000

[1] Includes the Company’s PEP.