Marketable Securities and Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities, available-for-sale	$ 120,804,000	$ 82,593,000
Other-than-temporary impairment loss	0	0	$ 0
Realized gains (losses)	0	0	$ 0
Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities, available-for-sale	$ 6,526,000	$ 21,561,000
Minimum \| Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, term	1 year
Maximum \| Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, term	2 years

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 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: 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=82897076&loc=d3e27161-111563

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+ References

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Amount of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI).

+ References

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

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Marketable Securities, Term

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