Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Other assets	$ 11,065	$ 1,602
Fair Value Measurements, Recurring Basis
Assets:
Cash and cash equivalents	99,800	62,500
Total assets measured at fair value	164,274	155,910
Other assets	6,100
Fair Value Measurements, Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured at fair value	99,808	61,512
Fair Value Measurements, Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured at fair value	58,336	94,398
Fair Value Measurements, Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets:
Total assets measured at fair value	6,130	0
Fair Value Measurements, Recurring Basis \| Money market funds
Assets:
Cash and cash equivalents	99,808	61,512
Fair Value Measurements, Recurring Basis \| Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	99,808	61,512
Fair Value Measurements, Recurring Basis \| Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Fair Value Measurements, Recurring Basis \| Money market funds \| Significant Unobservable Inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities
Assets:
Available-for-sale securities	3,996	3,990
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities	0	0
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities	3,996	3,990
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities	0	0
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises
Assets:
Available-for-sale securities	5,477	11,990
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities	0	0
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities	5,477	11,990
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities	0	0
Fair Value Measurements, Recurring Basis \| Corporate debt securities
Assets:
Available-for-sale securities	48,863	78,418
Fair Value Measurements, Recurring Basis \| Corporate debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities	0	0
Fair Value Measurements, Recurring Basis \| Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities	48,863	78,418
Fair Value Measurements, Recurring Basis \| Corporate debt securities \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities	0	$ 0
Fair Value Measurements, Recurring Basis \| Common stock warrants
Assets:
Available-for-sale securities	6,130
Fair Value Measurements, Recurring Basis \| Common stock warrants \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities	0
Fair Value Measurements, Recurring Basis \| Common stock warrants \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities	0
Fair Value Measurements, Recurring Basis \| Common stock warrants \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities	$ 6,130

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=99377789&loc=d3e19207-110258

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesCurrent
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

Amount of noncurrent assets classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_OtherAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_FairValueByAssetClassAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_WarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: