Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Amortized Cost	$ 130,842	$ 80,841
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(252)	(193)
Fair Value	130,594	80,648
Certificates of deposit
Investment [Line Items]
Amortized Cost	13,682	13,441
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	13,682	13,441
U.S. Treasury securities
Investment [Line Items]
Amortized Cost	31,850	14,787
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(107)	(71)
Fair Value	31,744	14,716
U.S. Agency securities
Investment [Line Items]
Amortized Cost	7,013	8,510
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(69)	(51)
Fair Value	6,944	8,459
Commercial paper
Investment [Line Items]
Amortized Cost	35,917	9,171
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	35,917	9,171
Asset-backed securities
Investment [Line Items]
Amortized Cost	8,992	3,026
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(1)	(1)
Fair Value	8,993	3,025
Corporate notes and bonds
Investment [Line Items]
Amortized Cost	33,388	31,906
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(75)	(70)
Fair Value	$ 33,314	$ 31,836

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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-Publisher FASB
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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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