Fair Value Measurements - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair value
Less than 12 months	$ 430,279	$ 67,217
12 months or greater	13,002	79,618
Total	443,281	146,835
Unrealized losses
Less than 12 months	(279)	(46)
12 months or greater	(2)	(45)
Total	(281)	(91)
U.S. government agency securities and treasuries
Fair value
Less than 12 months	363,028	13,234
12 months or greater	10,499	79,618
Total	373,527	92,852
Unrealized losses
Less than 12 months	(221)	(3)
12 months or greater	(2)	(45)
Total	(223)	(48)
Corporate bonds
Fair value
Less than 12 months	67,251	53,983
12 months or greater	2,503	0
Total	69,754	53,983
Unrealized losses
Less than 12 months	(58)	(43)
12 months or greater	0	0
Total	$ (58)	$ (43)