Financial Instruments (Details) $ in Thousands	9 Months Ended
Sep. 30, 2021 USD ($) position	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 928,596		$ 688,606
Proceeds from investments sold	77,600	$ 11,500
Other than temporary impairment	$ 0
Number of investments, unrealized loss position \| position	43
Amortized Cost
Less than or equal to one year	$ 403,211
Greater than one year but less than five years	523,885
Total	927,096
Fair Value
Less than or equal to one year	403,784
Greater than or equal to one year but less than five years	524,812
Total	928,596
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	982,799		713,337
Gross Unrealized Gain	2,199		4,310
Gross Unrealized Loss	(699)		(51)
Estimated Fair Value	984,299		717,596
Unrealized loss position	522,000
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	55,703		28,990
Estimated Fair Value	55,703		28,990
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	786,404		594,252
Gross Unrealized Gain	1,821		3,512
Gross Unrealized Loss	(350)		(20)
Estimated Fair Value	787,875		597,744
Corporate bonds and notes
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	52,868		12,331
Gross Unrealized Gain			2
Gross Unrealized Loss	(147)		(5)
Estimated Fair Value	52,721		12,328
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	87,824		77,764
Gross Unrealized Gain	378		796
Gross Unrealized Loss	(202)		(26)
Estimated Fair Value	88,000		78,534
Cash equivalents \| Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	55,703		28,990
Short-term investments \| Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 928,596		$ 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).