Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 325
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75031664&loc=d3e22529-158502

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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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This element represents the current portion of investments which are not defined as or included in marketable (debt, equity, or other) securities that are pledged or subject to withdrawal restrictions.

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No definition available.

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Cash Equivalents And Restricted Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	$ 808		$ 776
Gross realized gains or losses on cash equivalents	0	$ 0
Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	808		776
Maturity Date Within Three Months [Member] \| Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	246		537
Maturity Date Between Three Months And One Year [Member] \| Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	$ 562		$ 239
Minimum [Member] \| Maturity Date Between Three Months And One Year [Member] \| Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities maturity period	3 months		3 months
Maximum [Member] \| Maturity Date Within Three Months [Member] \| Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities maturity period	3 months		3 months
Maximum [Member] \| Maturity Date Between Three Months And One Year [Member] \| Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities maturity period	1 year		1 year