Fair Value of Financial Assets and Liabilities - By Type (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Marketable securities:
Amortized Cost	$ 102,549	$ 110,978
Gross Unrealized Gain	13
Gross Unrealized Loss	(4)	(25)
Fair Value	102,558	110,953
Corporate debt securities
Marketable securities:
Amortized Cost	16,833	5,030
Gross Unrealized Gain	5
Gross Unrealized Loss	(2)	(14)
Fair Value	16,836	5,016
Commercial paper
Marketable securities:
Amortized Cost	48,715	67,159
Fair Value	48,715	67,159
Asset-backed securities
Marketable securities:
Amortized Cost	22,075	21,745
Gross Unrealized Gain	8
Gross Unrealized Loss	(1)	(8)
Fair Value	22,082	21,737
U.S. government agency debt securities
Marketable securities:
Amortized Cost	14,926	17,044
Gross Unrealized Loss	(1)	(3)
Fair Value	$ 14,925	$ 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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