Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Fair Value Measurements
Investments in investment-grade corporate debt securities	$ 0	$ 0
Transfer of assets from Level 1 to Level 2 of the fair value measurement hierarchy	0
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Money market funds
Fair Value Measurements
Investments in money market funds	87,572,000	97,402,000
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Corporate debt securities
Fair Value Measurements
Investments in investment-grade corporate debt securities	0	0
Recurring basis \| Significant other observable inputs (Level 2) \| Money market funds
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis \| Significant other observable inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Investments in investment-grade corporate debt securities	17,031,000	29,425,000
Recurring basis \| Significant unobservable inputs (Level 3) \| Money market funds
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis \| Significant unobservable inputs (Level 3) \| Corporate debt securities
Fair Value Measurements
Investments in investment-grade corporate debt securities	$ 0	$ 0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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

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- 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

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- 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

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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 (bb)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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