Fair Value of Financial Assets and Liabilities - By Type (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable securities:
Amortized Cost	$ 61,514	$ 110,978
Gross Unrealized Gain	17
Gross Unrealized Loss	(1)	(25)
Fair Value	61,530	110,953
Corporate debt securities
Marketable securities:
Amortized Cost	14,359	5,030
Gross Unrealized Gain	13
Gross Unrealized Loss		(14)
Fair Value	14,372	5,016
Commercial paper
Marketable securities:
Amortized Cost	28,643	67,159
Gross Unrealized Loss	(1)
Fair Value	28,642	67,159
Asset-backed securities
Marketable securities:
Amortized Cost	5,998	21,745
Gross Unrealized Gain	4
Gross Unrealized Loss		(8)
Fair Value	6,002	21,737
U.S. government agency debt securities
Marketable securities:
Amortized Cost	12,514	17,044
Gross Unrealized Loss		(3)
Fair Value	$ 12,514	$ 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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