EMPLOYEE BENEFIT PLANS - Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
PLAN ASSETS
Fair value of plan assets	$ 94,897	$ 92,816	$ 90,748
(Level 1)
PLAN ASSETS
Fair value of plan assets	92,320	92,719
(Level 1) | Short-term investments.
PLAN ASSETS
Fair value of plan assets	2,745	3,488
(Level 1) | Mutual funds
PLAN ASSETS
Fair value of plan assets	68,804	69,539
(Level 1) | Domestic stock funds
PLAN ASSETS
Fair value of plan assets	18,361	18,377
(Level 1) | Bond funds
PLAN ASSETS
Fair value of plan assets	17,918	18,357
(Level 1) | Balanced funds
PLAN ASSETS
Fair value of plan assets	17,977	18,222
(Level 1) | International stock funds
PLAN ASSETS
Fair value of plan assets	14,548	14,583
(Level 1) | Domestic common stock
PLAN ASSETS
Fair value of plan assets	20,771	19,692
(Level 2)
PLAN ASSETS
Fair value of plan assets	2,461
(Level 2) | Domestic common stock
PLAN ASSETS
Fair value of plan assets	2,461
(Level 3)
PLAN ASSETS
Fair value of plan assets	116	97	$ 177
(Level 3) | Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	116	97
Total Measure at Fair Value
PLAN ASSETS
Fair value of plan assets	94,897	92,816
Total Measure at Fair Value | Short-term investments.
PLAN ASSETS
Fair value of plan assets	2,745	3,488
Total Measure at Fair Value | Mutual funds
PLAN ASSETS
Fair value of plan assets	68,804	69,539
Total Measure at Fair Value | Domestic stock funds
PLAN ASSETS
Fair value of plan assets	18,361	18,377
Total Measure at Fair Value | Bond funds
PLAN ASSETS
Fair value of plan assets	17,918	18,357
Total Measure at Fair Value | Balanced funds
PLAN ASSETS
Fair value of plan assets	17,977	18,222
Total Measure at Fair Value | International stock funds
PLAN ASSETS
Fair value of plan assets	14,548	14,583
Total Measure at Fair Value | Domestic common stock
PLAN ASSETS
Fair value of plan assets	23,232	19,692
Total Measure at Fair Value | Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	$ 116	$ 97