Fair Value Measurements - Assets Measured at Fair Value (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Measurements, Recurring
Investments:
Totals	$ 170,444,000	$ 195,864,000
Liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Totals	109,324,000	128,021,000
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Totals	61,120,000	67,843,000
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	50,242,000	41,434,000
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	50,242,000	41,434,000
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	50,242,000	41,434,000
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	48,300,000	67,843,000
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	48,300,000	67,843,000
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	59,082,000	53,758,000
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	59,082,000	53,758,000
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	12,820,000	32,829,000
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	32,829,000
Government agency securities \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	12,820,000	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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: