Financial Instruments and Fair Value Measurement - Recurring (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	$ 532,100	$ 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	16,661
Assets:
Cash and cash equivalents	898,013
Investments	71,158
Other current assets	28,662
Other assets	2,000
Total assets measured at fair value	1,016,494
Significant Other Observable Inputs (Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	533,200	$ 553,500
Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	28,865
Assets:
Other current assets	250
Total assets measured at fair value	29,115
Corporate debt securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	18,427
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	16,661
U.S. government and federal agency securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	10,438
Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	45,526
Assets:
Cash and cash equivalents	898,013
Investments	71,158
Other current assets	28,912
Other assets	2,000
Total assets measured at fair value	1,045,609
Total Measure at Fair Value | Corporate debt securities | Recurring basis
Short-term investments:
Short-term investments	18,427
Total Measure at Fair Value | U.S. government and federal agency securities | Recurring basis
Short-term investments:
Short-term investments	$ 27,099