Financial Instruments (Details) $ in Thousands, item in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) item position	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 493,852	$ 379,065
Other than temporary impairment	$ 0
Number of investments sold | item	11.5
Number of investments, unrealized loss position | position	9
Amortized Cost
Less than one year	$ 264,713
Greater than one year but less than five years	223,369
Total	488,082
Fair Value
Less than one year	265,534
Greater than one year but less than five years	228,318
Total	493,852
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	516,877	400,621
Unrealized Gain	5,801	1,008
Unrealized Loss	(31)	(87)
Estimated Fair Value	522,647	401,542
Unrealized loss position	117,800
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	28,795	22,477
Estimated Fair Value	28,795	22,477
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	399,622	292,506
Unrealized Gain	4,820	731
Unrealized Loss	(26)	(80)
Estimated Fair Value	404,416	293,157
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	88,460	85,638
Unrealized Gain	981	277
Unrealized Loss	(5)	(7)
Estimated Fair Value	89,436	85,908
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	28,795	22,477
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 493,852	$ 379,065