Cash Equivalents and Short-term and Long-term Investments - Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Cash and cash equivalents due in 90 days or less
Amortized Cost	$ 729,686	$ 237,677
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Estimated Fair Value	729,685	237,677	$ 169,542
Available-for-sale securities:
Amortized Cost	157,627	215,872
Gross Unrealized Gains	67	325
Gross Unrealized Losses	(18)	(28)
Debt Securities, Available-for-sale	157,676	216,169
Total cash, cash equivalents and available-for-sale securities
Amortized Cost	887,313	453,549
Gross Unrealized Gains	67	325
Gross Unrealized Losses	(19)	(28)
Estimated Fair Value	887,361	453,846
Corporate obligations
Available-for-sale securities:
Amortized Cost	39,162	124,676
Gross Unrealized Gains	57	219
Gross Unrealized Losses	(1)	(15)
Debt Securities, Available-for-sale	39,218	124,880
U.S. Treasury securities
Available-for-sale securities:
Amortized Cost	113,220	78,230
Gross Unrealized Gains	8	98
Gross Unrealized Losses	(16)	(1)
Debt Securities, Available-for-sale	113,212	78,327
Certificates of deposit
Available-for-sale securities:
Amortized Cost	245	490
Gross Unrealized Gains	2	3
Gross Unrealized Losses	0	0
Debt Securities, Available-for-sale	247	493
Mortgage and other asset backed securities
Available-for-sale securities:
Amortized Cost	5,000	12,476
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(1)	(12)
Debt Securities, Available-for-sale	$ 4,999	$ 12,469