Marketable Securities - Maturities of Held to Maturity Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Marketable Securities [Abstract]
Due within one year	$ 36,049
Due in one to five years	30,000
Total marketable securities	$ 66,049	$ 66,100	$ 107,266

Amount of investment in debt security measured at amortized cost (held-to-maturity).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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Amount of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next fiscal year through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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-Publisher FASB
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-Section 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

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Amount of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- References

No definition available.

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