Financial Instruments (Details) $ in Thousands	6 Months Ended
Jun. 30, 2021 USD ($) position	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 517,432		$ 688,606
Proceeds from investments sold	31,100	$ 11,500
Other than temporary impairment	$ 0
Number of investments, unrealized loss position \| position	23
Amortized Cost
Less than or equal to one year	$ 213,114
Greater than one year but less than five years	301,877
Total	514,991
Fair Value
Less than or equal to one year	213,493
Greater than or equal to one year but less than five years	303,939
Total	517,432
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	520,500		713,337
Unrealized Gain	2,687		4,310
Unrealized Loss	(246)		(51)
Estimated Fair Value	522,941		717,596
Unrealized loss position	158,800
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	5,509		28,990
Estimated Fair Value	5,509		28,990
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	432,610		594,252
Unrealized Gain	2,180		3,512
Unrealized Loss	(151)		(20)
Estimated Fair Value	434,639		597,744
Corporate bonds and notes
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	14,284		12,331
Unrealized Gain	2		2
Unrealized Loss	(38)		(5)
Estimated Fair Value	14,248		12,328
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	68,097		77,764
Unrealized Gain	505		796
Unrealized Loss	(57)		(26)
Estimated Fair Value	68,545		78,534
Cash equivalents \| Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	5,509		28,990
Short-term investments \| Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 517,432		$ 688,606

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).