Financial Instruments and Fair Value Measurement - Assets measured at fair value and Supplemental fair value information (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Sep. 30, 2015
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	$ 532,400	$ 531,500
Fixed-rate debt	540,000	540,000
Unsecured senior notes issued July 21, 2009
Supplemental fair value information about long-term fixed-rate debt
Fixed-rate debt	40,000	40,000
Unsecured senior notes issued March 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Fixed-rate debt	500,000	500,000
Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	49,565
Assets:
Cash and cash equivalents	907,032
Investments	87,050
Other current assets	26,813
Other assets	2,000
Total assets measured at fair value	1,072,460
Total Measure at Fair Value | Nonrecurring basis
Assets:
Assets held for sale	3,840
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Short-term investments:
Short-term investments	17,751
Assets:
Cash and cash equivalents	907,032
Investments	87,050
Other current assets	26,563
Other assets	2,000
Total assets measured at fair value	1,040,396
Significant Other Observable Inputs (Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	569,400	$ 553,500
Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	31,814
Assets:
Other current assets	250
Total assets measured at fair value	32,064
Significant Unobservable Inputs (Level 3) | Nonrecurring basis
Assets:
Assets held for sale	3,840
Certificate of deposit | Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	2,000
Certificate of deposit | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	2,000
Corporate debt securities | Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	20,608
Corporate debt securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	20,608
U.S. government and federal agency securities | Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	26,957
U.S. government and federal agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Short-term investments:
Short-term investments	17,751
U.S. government and federal agency securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	$ 9,206