Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash equivalents and marketable securities
Maximum maturity period of available-for-sale securities (in years)	3 years	3 years
Fair value	$ 441,186	$ 363,542
Amortized Cost	441,244	363,587
Gross unrealized gains	64	19
Gross unrealized losses	(122)	(64)
Cash, cash equivalents and marketable securities
Cash, fair value	61,463	44,122
Cash, amortized cost	61,463	44,122
Fair Value	502,649	407,664
Amortized Cost	502,707	407,709
Gross unrealized gains	64	19
Gross unrealized losses	(122)	(64)
Money market funds
Cash equivalents and marketable securities
Fair value	32,815	10,065
Amortized Cost	32,815	10,065
Gross unrealized gains	0	0
Gross unrealized losses	$ 0	$ 0
Weighted Rate of Return (as a percent)	0.01%	1.48%
U.S. Government bonds and notes
Cash equivalents and marketable securities
Fair value	$ 169,880	$ 39,086
Amortized Cost	169,920	39,087
Gross unrealized gains	3	0
Gross unrealized losses	$ (43)	$ (1)
Weighted Rate of Return (as a percent)	0.12%	1.49%
Corporate notes, bonds and commercial paper
Cash equivalents and marketable securities
Fair value	$ 238,491	$ 314,391
Amortized Cost	238,509	314,435
Gross unrealized gains	61	19
Gross unrealized losses	$ (79)	$ (63)
Weighted Rate of Return (as a percent)	0.21%	1.81%