Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) position	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 588,446	$ 688,606
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	22
Amortized Cost
Less than or equal to one year	$ 250,457
Greater than one year but less than five years	334,792
Total	585,249
Fair Value
Less than or equal to one year	251,028
Greater than or equal to one year but less than five years	337,418
Total	588,446
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	609,868	713,337
Unrealized Gain	3,367	4,310
Unrealized Loss	(170)	(51)
Estimated Fair Value	613,065	717,596
Unrealized loss position	114,100
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	24,619	28,990
Estimated Fair Value	24,619	28,990
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	492,814	594,252
Unrealized Gain	2,784	3,512
Unrealized Loss	(60)	(20)
Estimated Fair Value	495,538	597,744
Corporate bonds and notes
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	16,278	12,331
Unrealized Gain	1	2
Unrealized Loss	(31)	(5)
Estimated Fair Value	16,248	12,328
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	76,157	77,764
Unrealized Gain	582	796
Unrealized Loss	(79)	(26)
Estimated Fair Value	76,660	78,534
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	24,619	28,990
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 588,446	$ 688,606