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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 |

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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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