Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Amortized Cost	$ 80,841	$ 58,595
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(193)	(83)
Fair Value	80,648	58,519
Certificates of deposit
Investment [Line Items]
Amortized Cost	13,441	7,257
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	13,441	7,257
US Treasury securities
Investment [Line Items]
Amortized Cost	14,787	8,553
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(71)	(10)
Fair Value	14,716	8,544
US Agency securities
Investment [Line Items]
Amortized Cost	8,510	4,514
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(51)	(13)
Fair Value	8,459	4,501
Commercial paper
Investment [Line Items]
Amortized Cost	9,171
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	9,171
Asset-backed securities
Investment [Line Items]
Amortized Cost	3,026	5,554
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(1)	0
Fair Value	3,025	5,557
Corporate notes and bonds
Investment [Line Items]
Amortized Cost	31,906	32,717
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(70)	(60)
Fair Value	$ 31,836	$ 32,660

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- Definition

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
-Section 25
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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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