Financial Instruments and Fair Value Measurements - Measurements on Recurring Basis (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financial instruments and fair value measurements
Cash equivalents	$ 86,951	$ 166,253
Total	140,538	178,416
Level 1
Financial instruments and fair value measurements
Cash equivalents	86,951	166,253
Total	138,071	168,617
Level 2
Financial instruments and fair value measurements
Total	2,467	9,799
Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	49,996	53
Unrestricted available-for-sale, Debt securities \| Level 1
Financial instruments and fair value measurements
Investments	49,944
Unrestricted available-for-sale, Debt securities \| Level 2
Financial instruments and fair value measurements
Investments	52	53
Equity securities
Financial instruments and fair value measurements
Investments	2,415	9,746
Equity securities \| Level 2
Financial instruments and fair value measurements
Investments	2,415	9,746
Restricted
Financial instruments and fair value measurements
Investments	1,176	2,364
Restricted \| Level 1
Financial instruments and fair value measurements
Investments	1,176	2,364
Money market fund
Financial instruments and fair value measurements
Cash equivalents	86,951	21,043
Money market fund \| Level 1
Financial instruments and fair value measurements
Cash equivalents	86,951	21,043
Money market fund \| Restricted
Financial instruments and fair value measurements
Investments		114
Money market fund \| Restricted \| Level 1
Financial instruments and fair value measurements
Investments		114
Commercial paper
Financial instruments and fair value measurements
Cash equivalents		138,220
Commercial paper \| Level 1
Financial instruments and fair value measurements
Cash equivalents		138,220
U.S. Treasury Bills
Financial instruments and fair value measurements
Cash equivalents		6,990
U.S. Treasury Bills \| Level 1
Financial instruments and fair value measurements
Cash equivalents		6,990
U.S. Treasury Bills \| Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	49,944
U.S. Treasury Bills \| Unrestricted available-for-sale, Debt securities \| Level 1
Financial instruments and fair value measurements
Investments	49,944
Corporate debt securities \| Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	52	53
Corporate debt securities \| Unrestricted available-for-sale, Debt securities \| Level 2
Financial instruments and fair value measurements
Investments	52	53
Preferred stock \| Equity securities
Financial instruments and fair value measurements
Investments	2,415	9,746
Preferred stock \| Equity securities \| Level 2
Financial instruments and fair value measurements
Investments	2,415	9,746
Short-term bond \| Restricted
Financial instruments and fair value measurements
Investments	1,176	2,250
Short-term bond \| Restricted \| Level 1
Financial instruments and fair value measurements
Investments	$ 1,176	$ 2,250

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

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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

No definition available.

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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

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-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

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