Marketable Securities - Summary of Available-for-sale Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Marketable Securities [Line Items]
Marketable securities, Amortized Cost	$ 601,254	$ 176,627
Marketable securities, Unrealized Gain	1,226	56
Marketable securities, Unrealized Losses	(2,096)	(9)
Marketable securities, Fair Value	600,384	176,674
Commercial Debt Securities
Marketable Securities [Line Items]
Marketable securities, Amortized Cost	321,534	80,376
Marketable securities, Unrealized Gain	60	46
Marketable securities, Unrealized Losses	(2,096)	(5)
Marketable securities, Fair Value	319,498	80,417
Certificates of Deposit
Marketable Securities [Line Items]
Marketable securities, Amortized Cost	39,865
Marketable securities, Unrealized Gain	54
Marketable securities, Fair Value	39,919
US Government Agencies
Marketable Securities [Line Items]
Marketable securities, Amortized Cost	14,914
Marketable securities, Unrealized Gain	58
Marketable securities, Fair Value	14,972
U.S. Government Treasury Securities
Marketable Securities [Line Items]
Marketable securities, Amortized Cost	106,033	72,467
Marketable securities, Unrealized Gain	1,023	10
Marketable securities, Unrealized Losses		(4)
Marketable securities, Fair Value	107,056	72,473
Commercial Paper
Marketable Securities [Line Items]
Marketable securities, Amortized Cost	118,908	23,784
Marketable securities, Unrealized Gain	31
Marketable securities, Fair Value	$ 118,939	$ 23,784

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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Amount, before tax, of unrealized loss 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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