Short-Term Investments - Available-for-sale investments fair value (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2018	Apr. 01, 2017	Mar. 31, 2018	Apr. 01, 2017	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Investments in available-for-sale debt securities due in less than one year - Amortized Cost		$ 34,105		$ 34,105		$ 30,214
Investments in available-for-sale debt securities in one to five years - Amortized Cost	[1]	3,000		3,000		2,250
Investments in available-for-sale debt securities in less than one year - Estimated Fair Value		34,119		34,119		30,251
Investments in available-for-sale debt securities due in one to five years - Estimated Fair Value	[1]	3,008		3,008		$ 2,259
Proceeds from Sale of Available-for-sale Securities, Debt		$ 0	$ 0	$ 2,400	$ 100

[1] Classified as short-term investments because these securities are highly liquid and can be sold at any time.