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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, 2018
Sep. 30, 2017
Mar. 19, 2015
Supplemental fair value information about long-term fixed-rate debt      
Long-term debt $ 493,433 $ 492,902  
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 45,270    
Assets:      
Cash and cash equivalents 334,764    
Investments 58,381    
Other current assets 38,385    
Other assets 6,905    
Total assets measured at fair value 483,705    
Liabilities:      
Contingent earnout liability 15,702    
Carrying value      
Supplemental fair value information about long-term fixed-rate debt      
Long-term debt 493,400 492,900  
(Level 1) | Recurring basis      
Short-term investments:      
Short-term investments 25,914    
Assets:      
Cash and cash equivalents 334,764    
Investments 58,381    
Other current assets 38,385    
Other assets 6,905    
Total assets measured at fair value 464,349    
(Level 2)      
Supplemental fair value information about long-term fixed-rate debt      
Fair value of long-term fixed-rate debt 521,800 $ 529,000  
(Level 2) | Recurring basis      
Short-term investments:      
Short-term investments 19,356    
Assets:      
Total assets measured at fair value 19,356    
(Level 3) | Recurring basis      
Liabilities:      
Contingent earnout liability 15,702    
Certificates of deposit | Recurring basis      
Short-term investments:      
Short-term investments 3,498    
Certificates of deposit | (Level 2) | Recurring basis      
Short-term investments:      
Short-term investments 3,498    
Corporate and municipal debt securities | Recurring basis      
Short-term investments:      
Short-term investments 15,858    
Corporate and municipal debt securities | (Level 2) | Recurring basis      
Short-term investments:      
Short-term investments 15,858    
U.S. government and federal agency securities | Recurring basis      
Short-term investments:      
Short-term investments 25,914    
U.S. government and federal agency securities | (Level 1) | Recurring basis      
Short-term investments:      
Short-term investments $ 25,914