FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Long-term debt fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Mar. 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	$ 493,968	$ 492,902
Debt issued	500,000	500,000
Unsecured senior notes issued March 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Debt issued	500,000	500,000	$ 500,000
Carrying value
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	494,000	492,900
Total Measure at Fair Value | (Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	$ 509,300	$ 529,000