Marketable Securities (Tables)

12 Months Ended

Sep. 30, 2019

Investments Debt And Equity Securities [Abstract]

Fair Value of Available-for-Sale Marketable Securities by Type of Security

As of September 30, 2019 and 2018, the fair value of available-for-sale marketable securities, by type of security, was as follows:

	September 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
U.S. Treasury notes	$	170,519	$	60	$	(64	)	$	170,515
Corporate bonds		111,690		170		(23	)		111,837
Commercial paper		66,667		—		—			66,667
	$	348,876	$	230	$	(87	)	$	349,019

	September 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Commercial paper	$	113,885	$	—	$	—		$	113,885
Corporate bonds		105,105		1		(477	)		104,629
U.S. Treasury notes		42,801		—		(98	)		42,703
	$	261,791	$	1	$	(575	)	$	261,217