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Marketable Securities (Details)
$ in Thousands
12 Months Ended
Sep. 30, 2017
USD ($)
Security
Sep. 30, 2016
USD ($)
Sep. 30, 2015
USD ($)
Schedule of Available-for-sale Securities [Line Items]      
Number of securities below amortized cost basis 25    
Number of securities 44    
Proceeds from maturities of marketable securities | $ $ 87,105 $ 73,706 $ 38,028
Current Assets      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, next twelve months, maximum year mature 1 year    
Non-current Assets      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, year two through five, maximum year mature 2 years