FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENT - Long-term fixed-rate (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015	Mar. 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	$ 491,800
Unsecured senior notes issued March 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Debt issued	500,000		$ 500,000
Total Measure at Fair Value
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt		$ 531,500
Significant Other Observable Inputs (Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	529,600
Significant Other Observable Inputs (Level 2) | Total Measure at Fair Value
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt		$ 553,500
Recurring basis | Total Measure at Fair Value
Short-term investments:
Short-term investments	44,148
Assets:
Cash and cash equivalents	905,561
Investments	71,524
Other current assets	27,631
Other assets	2,000
Total assets measured at fair value	1,050,864
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Short-term investments:
Short-term investments	18,074
Assets:
Cash and cash equivalents	905,561
Investments	71,524
Other current assets	27,381
Other assets	2,000
Total assets measured at fair value	1,024,540
Recurring basis | Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	26,074
Assets:
Other current assets	250
Total assets measured at fair value	26,324
Recurring basis | Certificate of deposit | Total Measure at Fair Value
Short-term investments:
Short-term investments	2,000
Recurring basis | Certificate of deposit | Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	2,000
Recurring basis | Corporate debt securities | Total Measure at Fair Value
Short-term investments:
Short-term investments	18,591
Recurring basis | Corporate debt securities | Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	18,591
Recurring basis | U.S. government and federal agency securities | Total Measure at Fair Value
Short-term investments:
Short-term investments	23,557
Recurring basis | U.S. government and federal agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Short-term investments:
Short-term investments	18,074
Recurring basis | U.S. government and federal agency securities | Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	5,483
Nonrecurring basis | Total Measure at Fair Value
Assets:
Assets held for sale	1,106
Nonrecurring basis | Significant Unobservable Inputs (Level 3)
Assets:
Assets held for sale	$ 1,106