Marketable Securities (Tables)

6 Months Ended

Mar. 31, 2022

Investments, Debt and Equity Securities [Abstract]

Summary of Amortized Cost and Fair Value, Including Accrued Interest Receivable and Unrealized Holding Gains (Losses) on Short-term and Long-term Marketable Securities

The following is a summary of the amortized cost and the fair value, including accrued interest receivable as well as unrealized holding gains (losses) on the short-term and long-term marketable securities as of March 31, 2022 and September 30, 2021 (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized
	Cost		Losses		Gains		Fair Value
March 31, 2022:
U.S. Treasury securities and obligations of U.S. government agencies	$	720,648	$	(1,185)	$	18	$	719,481
Corporate securities		258,862		(1,899)		—		256,963
Municipal securities		100,518		(230)		—		100,288
	$	1,080,028	$	(3,314)	$	18	$	1,076,732
September 30, 2021:
Bank certificates of deposits	$	30	$	—	$	—	$	30
Corporate securities		3,624		—		—		3,624
Other debt securities		25		—		—		25
	$	3,679	$	—	$	—	$	3,679

Fair Value of Marketable Securities by Contractual Maturity

The fair values of the marketable securities by contractual maturities at March 31, 2022 are presented below (in thousands):

	Amortized
	Cost			Fair Value
Due in one year or less	$	817,223	$	816,512
Due after one year through five years		259,382		256,797
Due after five years through ten years		—		—
Due after ten years		3,422		3,423
Total marketable securities	$	1,080,027	$	1,076,732