Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) position	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 379,065	$ 107,461
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	8
Interest income	$ 4,100	1,800	$ 1,200
Amortized Cost
Less than one year	141,380
Greater than one year but less than five years	236,764
Total	378,144
Fair Value
Less than one year	141,552
Greater than one year but less than five years	237,513
Total	379,065
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	400,621	134,552
Unrealized Gain	1,008	36
Unrealized Loss	(87)	(588)
Estimated Fair Value	401,542	134,000
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	22,477	26,539
Estimated Fair Value	22,477	26,539
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	292,506	76,061
Unrealized Gain	731	29
Unrealized Loss	(80)	(405)
Estimated Fair Value	293,157	75,685
U.S. agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost		13,017
Unrealized Loss		(126)
Estimated Fair Value		12,891
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	85,638	18,935
Unrealized Gain	277	7
Unrealized Loss	(7)	(57)
Estimated Fair Value	85,908	18,885
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	22,477	26,539
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 379,065	$ 107,461