Fair Value Measurements - Assets Measured at Fair Value (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Measurements, Recurring
Investments:
Totals	$ 195,864,000	$ 255,351,000
Liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Totals	128,021,000	190,178,000
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Totals	67,843,000	65,173,000
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Investments:
Totals	0	0
Money market funds, included in cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	41,434,000	21,059,000
Money market funds, included in cash equivalents \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	41,434,000	21,059,000
Money market funds, included in cash equivalents \| Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	41,434,000	21,059,000
Money market funds, included in cash equivalents \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	0	0
Money market funds, included in cash equivalents \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	0	0
Corporate debt securities \| Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	67,843,000	65,173,000
Corporate debt securities \| Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Available-for-sale debt securities, fair value	0	0
Corporate debt securities \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	67,843,000	65,173,000
Corporate debt securities \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	0	0
U.S. Treasuries \| Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	53,758,000	110,948,000
U.S. Treasuries \| Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Available-for-sale debt securities, fair value	53,758,000	110,948,000
U.S. Treasuries \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	0	0
U.S. Treasuries \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	0	0
Government agency securities \| Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	32,829,000	58,171,000
Government agency securities \| Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Available-for-sale debt securities, fair value	32,829,000	58,171,000
Government agency securities \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	0	0
Government agency securities \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value of financial and nonfinancial obligations.

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