Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) item position	Mar. 31, 2019 item	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 331,681		$ 379,065
Other than temporary impairment	$ 0
Number of investments sold | item	0	0
Number of investments, unrealized loss position | position	10
Amortized Cost
Less than one year	$ 127,898
Greater than one year but less than five years	198,137
Total	326,035
Fair Value
Less than one year	128,675
Greater than one year but less than five years	203,006
Total	331,681
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	361,853		400,621
Unrealized Gain	5,688		1,008
Unrealized Loss	(42)		(87)
Estimated Fair Value	367,499		401,542
Unrealized loss position	23,200
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	35,818		22,477
Estimated Fair Value	35,818		22,477
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	243,821		292,506
Unrealized Gain	5,493		731
Unrealized Loss			(80)
Estimated Fair Value	249,314		293,157
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	82,214		85,638
Unrealized Gain	195		277
Unrealized Loss	(42)		(7)
Estimated Fair Value	82,367		85,908
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	35,818		22,477
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 331,681		$ 379,065