Investments (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Certificates of deposit	$ 2,160	$ 2,880
Certificates of deposit interest rate, lower range	1.70%
Certificates of deposit interest rate, upper range	1.95%
Investments held to maturity	$ 1,252	$ 6,077
Minimum
Schedule of Investments [Line Items]
Certificates of deposit maturity dates	1 month
Investments maturity dates	1 month
Maximum
Schedule of Investments [Line Items]
Certificates of deposit maturity dates	2 months
Investments maturity dates	2 months

Lower range of interest rates for certificates of deposit.

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- Definition

Upper range of interest rates for certificates of deposit.

+ References

No definition available.

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- Definition

Maturity dates of certificates of deposits.

+ References

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The perod of time during which held to maturity securities mature.

+ References

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- Definition

A savings certificate entitling the Entity (that is, bearer) to receive interest at an established maturity date, based upon a fixed interest rate. A certificate of deposit may be issued in any denomination. Certificates of deposit are generally issued by commercial banks and, therefore, insured by the FDIC (up to the prescribed limit). Certificates of deposit generally restrict holders from withdrawing funds on demand without the incurrence of penalties. Generally, only certificates of deposit with original maturities of three months or less qualify as cash equivalents. Original maturity means original maturity to the entity holding the investment. As a related example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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

+ References

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 5
-Subparagraph (a)
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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.

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