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EMPLOYEE BENEFIT PLANS - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 91,142	$ 94,897	$ 92,816
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	161	116	$ 97
Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	91,142	94,897
Fair Value | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	3,072	2,745
Fair Value | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	67,648	68,804
Fair Value | Domestic stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17,555	18,361
Fair Value | Bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	18,034	17,918
Fair Value | Balanced funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17,878	17,977
Fair Value | International stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	14,181	14,548
Fair Value | Domestic common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	20,261	23,232
Fair Value | Oil and gas properties
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	161	116
Fair Value | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	88,468	92,320
Fair Value | Level 1 | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	3,072	2,745
Fair Value | Level 1 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	67,648	68,804
Fair Value | Level 1 | Domestic stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17,555	18,361
Fair Value | Level 1 | Bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	18,034	17,918
Fair Value | Level 1 | Balanced funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17,878	17,977
Fair Value | Level 1 | International stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	14,181	14,548
Fair Value | Level 1 | Domestic common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17,748	20,771
Fair Value | Level 1 | Oil and gas properties
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	2,513	2,461
Fair Value | Level 2 | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2 | Domestic stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2 | Bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2 | Balanced funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2 | International stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 2 | Domestic common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	2,513	2,461
Fair Value | Level 2 | Oil and gas properties
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	161	116
Fair Value | Level 3 | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | Domestic stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | Bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | Balanced funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | International stock funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | Domestic common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Fair Value | Level 3 | Oil and gas properties
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 161	$ 116

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