Cash and Marketable Securities - Summary of Available for Sale Securities (Details) $ in Thousands	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 630,133	$ 375,094
Unrealized Holding Gains	377	729
Unrealized Holding Losses	(15)	(30)
Aggregate Fair Value	630,495	375,793
Short-term marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	609,791	335,231
Unrealized Holding Gains	377	689
Unrealized Holding Losses	(14)	(13)
Aggregate Fair Value	610,154	335,907
Short-term marketable securities \| U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	542,479	249,478
Unrealized Holding Gains	161	594
Unrealized Holding Losses	(3)	(2)
Aggregate Fair Value	$ 542,637	$ 250,070
Number of securities held in unrealized holding loss position \| security	8	2
Aggregate fair value, unrealized holding loss position	$ 73,300	$ 9,300
Short-term marketable securities \| U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,122	1,999
Unrealized Holding Gains	0	1
Unrealized Holding Losses	(11)	0
Aggregate Fair Value	$ 10,111	2,000
Number of securities held in unrealized holding loss position \| security	2
Aggregate fair value, unrealized holding loss position	$ 10,100
Short-term marketable securities \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	43,703	53,396
Unrealized Holding Gains	216	94
Unrealized Holding Losses	0	(11)
Aggregate Fair Value	43,919	$ 53,479
Number of securities held in unrealized holding loss position \| security		4
Aggregate fair value, unrealized holding loss position		$ 25,900
Short-term marketable securities \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,487	30,358
Unrealized Holding Gains	0	0
Unrealized Holding Losses	0	0
Aggregate Fair Value	13,487	30,358
Long-term marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,342	39,863
Unrealized Holding Gains	0	40
Unrealized Holding Losses	(1)	(17)
Aggregate Fair Value	20,341	39,886
Long-term marketable securities \| U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,153	13,865
Unrealized Holding Gains	0	6
Unrealized Holding Losses	0	(2)
Aggregate Fair Value	5,153	$ 13,869
Number of securities held in unrealized holding loss position \| security		1
Aggregate fair value, unrealized holding loss position		$ 10,100
Long-term marketable securities \| U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,189
Unrealized Holding Gains	0
Unrealized Holding Losses	(1)
Aggregate Fair Value	$ 15,188
Number of securities held in unrealized holding loss position \| security	1
Aggregate fair value, unrealized holding loss position	$ 15,200
Long-term marketable securities \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		25,998
Unrealized Holding Gains		34
Unrealized Holding Losses		(15)
Aggregate Fair Value		$ 26,017
Number of securities held in unrealized holding loss position \| security		3
Aggregate fair value, unrealized holding loss position		$ 17,200

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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