Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Total	$ 119,228	$ 80,841
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(248)	(193)
Fair Value	118,982	80,648
Certificates of deposit
Investment [Line Items]
Total	12,481	13,441
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	12,481	13,441
U.S. Treasury securities
Investment [Line Items]
Total	34,390	14,787
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(122)	(71)
Fair Value	34,268	14,716
U.S. Agency securities
Investment [Line Items]
Total	8,017	8,510
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(66)	(51)
Fair Value	7,951	8,459
Commercial paper
Investment [Line Items]
Total	18,456	9,171
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	18,456	9,171
Asset-backed securities
Investment [Line Items]
Total	11,968	3,026
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	11,967	3,025
Corporate notes and bonds
Investment [Line Items]
Total	33,916	31,906
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(59)	(70)
Fair Value	$ 33,859	$ 31,836

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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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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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Amount 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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