Fair Value Measurements - Assets Measured at Fair Value (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Measurements, Recurring
Investments:
Totals	$ 148,574,000	$ 170,444,000
Liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Totals	106,746,000	109,324,000
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Totals	41,828,000	61,120,000
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Investments:
Totals	0	0
Corporate debt securities \| Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	33,366,000	48,300,000
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	33,366,000	48,300,000
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	40,755,000	59,082,000
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	40,755,000	59,082,000
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	8,462,000	12,820,000
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	0	0
Government agency securities \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	8,462,000	12,820,000
Government agency securities \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	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	65,991,000	50,242,000
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	65,991,000	50,242,000
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	65,991,000	50,242,000
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

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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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 326
-SubTopic 30
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Fair value of financial and nonfinancial obligations.

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