Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($) position
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 688,606	$ 379,065
Amount of investments sold	$ 30,100	$ 0
Number of investments, unrealized loss position | position	13	13
Unrealized loss position	$ 144,000
Amortized Cost
Less than or equal to one year	322,416
Greater than one year but less than five years	361,931
Total	684,347
Fair Value
Less than or equal to one year	323,002
Greater than or equal to one year but less than five years	365,604
Total	688,606
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	713,337	$ 400,621
Unrealized Gain	4,310	1,008
Unrealized Loss	(51)	(87)
Estimated Fair Value	717,596	401,542
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	28,990	22,477
Estimated Fair Value	28,990	22,477
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	594,252	292,506
Unrealized Gain	3,512	731
Unrealized Loss	(20)	(80)
Estimated Fair Value	597,744	293,157
Corporate bonds and notes
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	12,331
Unrealized Gain	2
Unrealized Loss	(5)
Estimated Fair Value	12,328
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	77,764	85,638
Unrealized Gain	796	277
Unrealized Loss	(26)	(7)
Estimated Fair Value	78,534	85,908
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	28,990	22,477
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 688,606	$ 379,065