DEBT - Summary of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Long-term debt	$ 1,298,136	$ 1,227,919
Senior Notes \| 6.5% Senior Notes
Debt Instrument [Line Items]
6.5% Senior Notes due 2020 (Senior Notes) aggregate principal amount	448,650	448,650
Senior Notes unamortized premium	2,568	3,245
Senior Notes unamortized debt issuance costs	(3,082)	(3,976)
Long-term debt	$ 448,136	447,919
Stated interest rate	6.50%
Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt	$ 850,000	$ 780,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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