Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable securities:
Amortized Cost Basis	$ 161,777
Unrealized Gains	328
Unrealized Losses	(25)
Fair Value	162,080	$ 89,965
US Government
Marketable securities:
Amortized Cost Basis	77,067
Unrealized Gains	163
Unrealized Losses	(21)
Fair Value	77,209
Corporate Bonds
Marketable securities:
Amortized Cost Basis	63,330
Unrealized Gains	155
Unrealized Losses	(4)
Fair Value	63,481
Commercial Paper
Marketable securities:
Amortized Cost Basis	7,981
Unrealized Gains	3
Fair Value	7,984
Money Market Funds
Marketable securities:
Amortized Cost Basis	6,528
Fair Value	6,528
Certification Of Deposit
Marketable securities:
Amortized Cost Basis	6,871
Unrealized Gains	7
Fair Value	$ 6,878

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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