Cash and Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 375,094	$ 535,705
Unrealized Holding Gains	729	369
Unrealized Holding Losses	(30)	(1,019)
Aggregate Fair Value	375,793	535,055
Short-term marketable securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	335,231	387,988
Unrealized Holding Gains	689	30
Unrealized Holding Losses	(13)	(844)
Aggregate Fair Value	335,907	387,174
Short-term marketable securities: \| U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	249,478	220,081
Unrealized Holding Gains	594	29
Unrealized Holding Losses	(2)	(356)
Aggregate Fair Value	250,070	219,754
Short-term marketable securities: \| U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,999	73,373
Unrealized Holding Gains	1	0
Unrealized Holding Losses	0	(222)
Aggregate Fair Value	2,000	73,151
Short-term marketable securities: \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,396	71,940
Unrealized Holding Gains	94	1
Unrealized Holding Losses	(11)	(266)
Aggregate Fair Value	53,479	71,675
Short-term marketable securities: \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30,358	22,594
Unrealized Holding Gains	0	0
Unrealized Holding Losses	0	0
Aggregate Fair Value	30,358	22,594
Long-term marketable securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	39,863	147,717
Unrealized Holding Gains	40	339
Unrealized Holding Losses	(17)	(175)
Aggregate Fair Value	39,886	147,881
Long-term marketable securities: \| U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,865	116,878
Unrealized Holding Gains	6	329
Unrealized Holding Losses	(2)	(76)
Aggregate Fair Value	13,869	117,131
Long-term marketable securities: \| U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,975
Unrealized Holding Gains		2
Unrealized Holding Losses		0
Aggregate Fair Value		1,977
Long-term marketable securities: \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,998	28,864
Unrealized Holding Gains	34	8
Unrealized Holding Losses	(15)	(99)
Aggregate Fair Value	$ 26,017	$ 28,773

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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