Investments and Fair Value Measurements - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 74,695
Gross Unrealized Holding Gains	2
Gross Unrealized Holding Losses	(90)
Fair Value	74,607
Cash equivalents	6,661	$ 258
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,426	70,369
Gross Unrealized Holding Gains	2	0
Gross Unrealized Holding Losses	(90)	(91)
Fair Value	73,338	70,278
Short-term investments \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,102	16,755
Gross Unrealized Holding Gains	1	0
Gross Unrealized Holding Losses	(2)	(14)
Fair Value	8,101	16,741
Short-term investments \| Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	65,324	53,367
Gross Unrealized Holding Gains	1	0
Gross Unrealized Holding Losses	(88)	(77)
Fair Value	65,237	53,290
Short-term investments \| Municipal notes and bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		247
Gross Unrealized Holding Gains		0
Gross Unrealized Holding Losses		0
Fair Value		247
Long-term investments \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,269
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	0
Fair Value	1,269
Long-term investments \| Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,269
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	0
Fair Value	$ 1,269
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Cash equivalents		$ 258

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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss 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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-Publisher FASB
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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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-Publisher FASB
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