EMPLOYEE BENEFIT PLANS - Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
PLAN ASSETS
Fair value of plan assets	$ 90,748	$ 98,060	$ 108,157
Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	177	387	$ 301
Quoted Prices in Active Markets for Identical Assets (Level 1)
PLAN ASSETS
Fair value of plan assets	90,571	97,673
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments.
PLAN ASSETS
Fair value of plan assets	467	2,248
Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
PLAN ASSETS
Fair value of plan assets	70,250	78,060
Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic stock funds
PLAN ASSETS
Fair value of plan assets	36,107	40,072
Quoted Prices in Active Markets for Identical Assets (Level 1) | Bond funds
PLAN ASSETS
Fair value of plan assets	22,809	25,344
Quoted Prices in Active Markets for Identical Assets (Level 1) | International stock funds
PLAN ASSETS
Fair value of plan assets	11,334	12,644
Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic common stock
PLAN ASSETS
Fair value of plan assets	18,305	15,883
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity stock
PLAN ASSETS
Fair value of plan assets	1,549	1,482
Significant Unobservable Inputs (Level 3)
PLAN ASSETS
Fair value of plan assets	177	387
Significant Unobservable Inputs (Level 3) | Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	177	387
Total Measure at Fair Value
PLAN ASSETS
Fair value of plan assets	90,748	98,060
Total Measure at Fair Value | Short-term investments.
PLAN ASSETS
Fair value of plan assets	467	2,248
Total Measure at Fair Value | Mutual funds
PLAN ASSETS
Fair value of plan assets	70,250	78,060
Total Measure at Fair Value | Domestic stock funds
PLAN ASSETS
Fair value of plan assets	36,107	40,072
Total Measure at Fair Value | Bond funds
PLAN ASSETS
Fair value of plan assets	22,809	25,344
Total Measure at Fair Value | International stock funds
PLAN ASSETS
Fair value of plan assets	11,334	12,644
Total Measure at Fair Value | Domestic common stock
PLAN ASSETS
Fair value of plan assets	18,305	15,883
Total Measure at Fair Value | Foreign equity stock
PLAN ASSETS
Fair value of plan assets	1,549	1,482
Total Measure at Fair Value | Oil and Gas Properties
PLAN ASSETS
Fair value of plan assets	$ 177	$ 387