Financial Instruments and Fair Value Measurement - Recurring (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	$ 531,800	$ 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
Unsecured senior notes issued March 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Fixed-rate debt	500,000	500,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Short-term investments:
Short-term investments	8,716
Assets:
Cash and cash equivalents	848,230
Investments	73,528
Other current assets	37,005
Other assets	2,000
Total assets measured at fair value	969,479
Significant Other Observable Inputs (Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	540,400	$ 553,500
Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	38,992
Assets:
Other current assets	250
Total assets measured at fair value	39,242
Certificate of deposit | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	2,100
Corporate debt securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	25,450
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	8,716
U.S. government and federal agency securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	11,442
Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	47,708
Assets:
Cash and cash equivalents	848,230
Investments	73,528
Other current assets	37,255
Other assets	2,000
Total assets measured at fair value	1,008,721
Total Measure at Fair Value | Certificate of deposit | Recurring basis
Short-term investments:
Short-term investments	2,100
Total Measure at Fair Value | Corporate debt securities | Recurring basis
Short-term investments:
Short-term investments	25,450
Total Measure at Fair Value | U.S. government and federal agency securities | Recurring basis
Short-term investments:
Short-term investments	$ 20,158