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Financial Instruments and Fair Value Measurement - Assets measured at fair value and Supplemental fair value information (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Sep. 30, 2016
Mar. 19, 2015
Supplemental fair value information about long-term fixed-rate debt      
Carrying value of long-term fixed-rate debt $ 492,373 $ 491,847  
Unsecured senior notes issued March 19, 2015      
Supplemental fair value information about long-term fixed-rate debt      
Debt issued 500,000   $ 500,000
Recurring basis      
Short-term investments:      
Short-term investments 48,011    
Assets:      
Cash and cash equivalents 741,746    
Investments 74,632    
Other current assets 28,344    
Other assets 2,000    
Total assets measured at fair value 894,733    
Carrying value      
Supplemental fair value information about long-term fixed-rate debt      
Carrying value of long-term fixed-rate debt 492,400 491,800  
(Level 1) | Recurring basis      
Short-term investments:      
Short-term investments 25,195    
Assets:      
Cash and cash equivalents 741,746    
Investments 74,632    
Other current assets 28,094    
Other assets 2,000    
Total assets measured at fair value 871,667    
(Level 2) | Recurring basis      
Short-term investments:      
Short-term investments 22,816    
Assets:      
Other current assets 250    
Total assets measured at fair value 23,066    
(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 523,700 $ 529,600  
Certificate of deposit | Recurring basis      
Short-term investments:      
Short-term investments 3,099    
Certificate of deposit | (Level 2) | Recurring basis      
Short-term investments:      
Short-term investments 3,099    
Corporate and municipal debt securities | Recurring basis      
Short-term investments:      
Short-term investments 15,727    
Corporate and municipal debt securities | (Level 2) | Recurring basis      
Short-term investments:      
Short-term investments 15,727    
U.S. government and federal agency securities | Recurring basis      
Short-term investments:      
Short-term investments 29,185    
U.S. government and federal agency securities | (Level 1) | Recurring basis      
Short-term investments:      
Short-term investments 25,195    
U.S. government and federal agency securities | (Level 2) | Recurring basis      
Short-term investments:      
Short-term investments $ 3,990