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EMPLOYEE BENEFIT PLANS - Plan assets (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | $ 90,748 | $ 98,060 |
Fair value of plan assets at end of year | $ 92,816 | 90,748 | $ 98,060
DEFINED CONTRIBUTION PLAN |  |  |
Percentage of employer's contribution under 401(k)/ Thrift Plan matching the first 5 percent of participant's compensation subject to certain limitations | 100.00% |  |
Percentage of participant's compensation eligible for employer's matching contribution | 5.00% |  |
Annual expense incurred for defined contribution plan | $ 16,600 | 21,600 | 24,800
General and Administrative |  |  |
DEFINED CONTRIBUTION PLAN |  |  |
Employer contribution |  | 6,300 |
Oil and Gas Properties |  |  |
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 177 | 387 |
Unrealized losses relating to property still held at the reporting date | (80) | (210) |
Fair value of plan assets at end of year | $ 97 | $ 177 | $ 387

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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held. + 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 (d)(5)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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