EMPLOYEE BENEFIT PLANS - Plan assets (Details) - USD ($) $ in Thousands	12 Months Ended
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

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

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

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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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

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- References

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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

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- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

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