Financial Instruments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) position	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 600,205	$ 379,065
Other than temporary impairment	0
Amount of investments sold	$ 11,500
Number of investments, unrealized loss position | position	8
Amortized Cost
Less than or equal to one year	$ 338,583
Greater than one year but less than five years	256,464
Total	595,047
Fair Value
Less than one year	339,655
Greater than or equal to one year but less than five years	260,550
Total	600,205
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	773,948	400,621
Unrealized Gain	5,170	1,008
Unrealized Loss	(19)	(87)
Estimated Fair Value	779,099	401,542
Unrealized loss position	122,900
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	178,894	22,477
Estimated Fair Value	178,894	22,477
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	526,643	292,506
Unrealized Gain	4,173	731
Unrealized Loss	(8)	(80)
Estimated Fair Value	530,808	293,157
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	68,411	85,638
Unrealized Gain	997	277
Unrealized Loss	(11)	(7)
Estimated Fair Value	69,397	85,908
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	178,894	22,477
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 600,205	$ 379,065