Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2021

Financial Instruments

Schedule of available-for-sale securities

	March 31, 2021								December 31, 2020
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Estimated Fair Value		Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Estimated Fair Value
	(in thousands)
Money market deposits	$	24,619	$	—	$	—	$	24,619	$	28,990	$	—	$	—	$	28,990
U.S. Treasury securities (1)		492,814		2,784		(60)		495,538		594,252		3,512		(20)		597,744
Corporate bonds and notes (1)		16,278		1		(31)		16,248		12,331		2		(5)		12,328
Municipal securities		76,157		582		(79)		76,660		77,764		796		(26)		78,534
Total	$	609,868	$	3,367	$	(170)	$	613,065	$	713,337	$	4,310	$	(51)	$	717,596
Classified as:
Cash equivalents (2)							$	24,619							$	28,990
Short-term investments								588,446								688,606
Total							$	613,065							$	717,596

Summarized portfolio of available-for-sale securities by contractual maturity

	March 31, 2021
	Amortized Cost		Fair Value
	(in thousands)
Less than or equal to one year	$	250,457	$	251,028
Greater than one year but less than five years		334,792		337,418
Total	$	585,249	$	588,446