Fair Value of Financial Assets and Liabilities - By Type (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Marketable securities:
Amortized Cost	$ 83,824	$ 110,978
Gross Unrealized Gain	39
Gross Unrealized Loss		(25)
Fair Value	83,863	110,953
Corporate debt securities
Marketable securities:
Amortized Cost	16,872	5,030
Gross Unrealized Gain	20
Gross Unrealized Loss		(14)
Fair Value	16,892	5,016
Commercial paper
Marketable securities:
Amortized Cost	25,834	67,159
Fair Value	25,834	67,159
Asset-backed securities
Marketable securities:
Amortized Cost	18,646	21,745
Gross Unrealized Gain	9
Gross Unrealized Loss		(8)
Fair Value	18,655	21,737
U.S. government agency debt securities
Marketable securities:
Amortized Cost	22,472	17,044
Gross Unrealized Gain	10
Gross Unrealized Loss		(3)
Fair Value	$ 22,482	$ 17,041

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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