FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Long-Term Debt Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	$ 491,651	$ 493,968
(Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	530,000	509,300
Carrying value
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	$ 491,700	$ 494,000

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LongTermDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant