EMPLOYEE BENEFIT PLANS - Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
PLAN ASSETS
Fair value of plan assets	$ 92,816	$ 90,748	$ 98,060
Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	97	177	$ 387
(Level 1)
PLAN ASSETS
Fair value of plan assets	92,719	90,571
(Level 1) | Short-term investments.
PLAN ASSETS
Fair value of plan assets	3,488	467
(Level 1) | Mutual funds
PLAN ASSETS
Fair value of plan assets	69,539	70,250
(Level 1) | Domestic stock funds
PLAN ASSETS
Fair value of plan assets	18,377	36,107
(Level 1) | Bond funds
PLAN ASSETS
Fair value of plan assets	18,357	22,809
(Level 1) | Balanced funds
PLAN ASSETS
Fair value of plan assets	18,222
(Level 1) | International stock funds
PLAN ASSETS
Fair value of plan assets	14,583	11,334
(Level 1) | Domestic common stock
PLAN ASSETS
Fair value of plan assets	19,692	18,305
(Level 1) | Foreign equity stock
PLAN ASSETS
Fair value of plan assets		1,549
(Level 3)
PLAN ASSETS
Fair value of plan assets	97	177
(Level 3) | Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	97	177
Total Measure at Fair Value
PLAN ASSETS
Fair value of plan assets	92,816	90,748
Total Measure at Fair Value | Short-term investments.
PLAN ASSETS
Fair value of plan assets	3,488	467
Total Measure at Fair Value | Mutual funds
PLAN ASSETS
Fair value of plan assets	69,539	70,250
Total Measure at Fair Value | Domestic stock funds
PLAN ASSETS
Fair value of plan assets	18,377	36,107
Total Measure at Fair Value | Bond funds
PLAN ASSETS
Fair value of plan assets	18,357	22,809
Total Measure at Fair Value | Balanced funds
PLAN ASSETS
Fair value of plan assets	18,222
Total Measure at Fair Value | International stock funds
PLAN ASSETS
Fair value of plan assets	14,583	11,334
Total Measure at Fair Value | Domestic common stock
PLAN ASSETS
Fair value of plan assets	19,692	18,305
Total Measure at Fair Value | Foreign equity stock
PLAN ASSETS
Fair value of plan assets		1,549
Total Measure at Fair Value | Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	$ 97	$ 177