Financial Instruments and Fair Value Measurements - Measurements on Recurring Basis (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial instruments and fair value measurements
Cash equivalents	$ 173,787	$ 180,930
Total	224,118	229,452
Level 1
Financial instruments and fair value measurements
Cash equivalents	173,787	180,930
Total	193,849	201,769
Level 2
Financial instruments and fair value measurements
Total	30,269	27,683
Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	9,817	8,958
Unrestricted available-for-sale, Debt securities \| Level 1
Financial instruments and fair value measurements
Investments	6,979	6,937
Unrestricted available-for-sale, Debt securities \| Level 2
Financial instruments and fair value measurements
Investments	2,838	2,021
Restricted
Financial instruments and fair value measurements
Investments	2,328	3,432
Restricted \| Level 1
Financial instruments and fair value measurements
Investments	2,328	3,432
Money market fund
Financial instruments and fair value measurements
Cash equivalents	5,711	43,346
Money market fund \| Level 1
Financial instruments and fair value measurements
Cash equivalents	5,711	43,346
Money market fund \| Restricted
Financial instruments and fair value measurements
Investments	113	167
Money market fund \| Restricted \| Level 1
Financial instruments and fair value measurements
Investments	113	167
Commercial paper
Financial instruments and fair value measurements
Cash equivalents	168,076	107,586
Commercial paper \| Level 1
Financial instruments and fair value measurements
Cash equivalents	168,076	107,586
U.S. Treasury securities
Financial instruments and fair value measurements
Cash equivalents		29,998
U.S. Treasury securities \| Level 1
Financial instruments and fair value measurements
Cash equivalents		29,998
U.S. Treasury securities \| Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	6,979	6,937
U.S. Treasury securities \| Unrestricted available-for-sale, Debt securities \| Level 1
Financial instruments and fair value measurements
Investments	6,979	6,937
Corporate debt securities \| Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	2,838	2,021
Corporate debt securities \| Unrestricted available-for-sale, Debt securities \| Level 2
Financial instruments and fair value measurements
Investments	2,838	2,021
Preferred stock \| Equity securities
Financial instruments and fair value measurements
Investments	38,186	36,132
Preferred stock \| Equity securities \| Level 1
Financial instruments and fair value measurements
Investments	10,755	10,470
Preferred stock \| Equity securities \| Level 2
Financial instruments and fair value measurements
Investments	27,431	25,662
Short-term bond \| Restricted
Financial instruments and fair value measurements
Investments	2,215	3,265
Short-term bond \| Restricted \| Level 1
Financial instruments and fair value measurements
Investments	$ 2,215	$ 3,265

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

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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

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=99377789&loc=d3e19207-110258

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