Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2020

Financial Instruments

Schedule of available-for-sale securities

	December 31, 2020								December 31, 2019
	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	$	28,990	$	—	$	—	$	28,990	$	22,477	$	—	$	—	$	22,477
U.S. Treasury securities (1)		594,252		3,512		(20)		597,744		292,506		731		(80)		293,157
Corporate bonds and notes (1)		12,331		2		(5)		12,328		—		—		—		—
Municipal securities		77,764		796		(26)		78,534		85,638		277		(7)		85,908
Total	$	713,337	$	4,310	$	(51)	$	717,596	$	400,621	$	1,008	$	(87)	$	401,542
Classified as:
Cash equivalents (2)							$	28,990							$	22,477
Short-term investments								688,606								379,065
Total							$	717,596							$	401,542

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

	December 31, 2020
	Amortized Cost		Fair Value
	(in thousands)
Less than or equal to one year	$	322,416	$	323,002
Greater than one year but less than five years		361,931		365,604
Total	$	684,347	$	688,606