Marketable Securities - Schedule of Available-for-sale Marketable Securities and Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Cash and cash equivalents:
Amortized Cost Value	$ 66,901	$ 59,321
Fair Value	66,901	59,321
Marketable securities:
Amortized Cost Value	121,621	177,895
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(45)	(755)
Fair Value	121,576	177,140
Domestic corporate debt securities
Marketable securities:
Amortized Cost Value	74,752	132,886
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9)	(530)
Fair Value	74,743	132,356
Domestic corporate commercial paper
Marketable securities:
Amortized Cost Value	16,841
Gross Unrealized Gains	0
Gross Unrealized Losses	(11)
Fair Value	16,830
Agency bonds
Marketable securities:
Amortized Cost Value	30,028	45,009
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(25)	(225)
Fair Value	30,003	44,784
Cash
Cash and cash equivalents:
Amortized Cost Value	63,660	20,448
Fair Value	63,660	20,448
Money market funds
Cash and cash equivalents:
Amortized Cost Value	3,241	38,873
Fair Value	$ 3,241	$ 38,873

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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Amortized cost of 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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