Financial Instruments and Fair Value Measurements - Measurements on Recurring Basis (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financial instruments and fair value measurements
Cash equivalents	$ 189,875	$ 170,475
Total	246,313	286,212
Level 1
Financial instruments and fair value measurements
Cash equivalents	189,875	170,475
Total	214,089	195,531
Level 2
Financial instruments and fair value measurements
Total	32,224	90,681
Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	14,202	76,245
Unrestricted available-for-sale, Debt securities \| Level 1
Financial instruments and fair value measurements
Investments	9,946	9,870
Unrestricted available-for-sale, Debt securities \| Level 2
Financial instruments and fair value measurements
Investments	4,256	66,375
Restricted
Financial instruments and fair value measurements
Investments	3,406	4,469
Restricted \| Level 1
Financial instruments and fair value measurements
Investments	3,406	4,469
Money market fund
Financial instruments and fair value measurements
Cash equivalents	24,256	10,505
Money market fund \| Level 1
Financial instruments and fair value measurements
Cash equivalents	24,256	10,505
Money market fund \| Restricted
Financial instruments and fair value measurements
Investments	167	218
Money market fund \| Restricted \| Level 1
Financial instruments and fair value measurements
Investments	167	218
Commercial paper
Financial instruments and fair value measurements
Cash equivalents	165,619	159,970
Commercial paper \| Level 1
Financial instruments and fair value measurements
Cash equivalents	165,619	159,970
U.S. Treasury securities \| Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	9,946	9,870
U.S. Treasury securities \| Unrestricted available-for-sale, Debt securities \| Level 1
Financial instruments and fair value measurements
Investments	9,946	9,870
Corporate debt securities \| Unrestricted available-for-sale, Debt securities
Financial instruments and fair value measurements
Investments	4,256	66,375
Corporate debt securities \| Unrestricted available-for-sale, Debt securities \| Level 2
Financial instruments and fair value measurements
Investments	4,256	66,375
Preferred stock \| Equity securities
Financial instruments and fair value measurements
Investments	38,830	35,023
Preferred stock \| Equity securities \| Level 1
Financial instruments and fair value measurements
Investments	10,862	10,717
Preferred stock \| Equity securities \| Level 2
Financial instruments and fair value measurements
Investments	27,968	24,306
Short-term bond \| Restricted
Financial instruments and fair value measurements
Investments	3,239	4,251
Short-term bond \| Restricted \| Level 1
Financial instruments and fair value measurements
Investments	$ 3,239	$ 4,251

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&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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+ References

No definition available.

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24584-111560

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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