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Investments - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Unrealized net (losses) gains on available-for-sale securities	$ 255	$ 430
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of investments	One year or less

X

- Definition

Investment maturity period.

+ References

No definition available.

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- Definition

The change in unrealized holding gain (loss), net of tax, on available-for-sale securities included in a separate component of shareholders' equity during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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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.

+ References

No definition available.

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