Marketable Securities (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of securities below amortized cost basis	28
Number of securities	29
Proceeds from maturities and sales of marketable securities | $	$ 29,513	$ 57,039
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