Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Less Than 12 Months, Market Value	$ 39,966	$ 31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses	295	149
Available for sale securities, 12 Months or Greater, Market Value	14,292	22,610
Available for sale securities, 12 Months or Greater, Unrealized Losses	47	39
Federal Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, 12 Months or Greater, Market Value	1,794	1,793
Available for sale securities, 12 Months or Greater, Unrealized Losses	6	7
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Less Than 12 Months, Market Value	39,966	31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses	295	149
Available for sale securities, 12 Months or Greater, Market Value	12,498	20,817
Available for sale securities, 12 Months or Greater, Unrealized Losses	$ 41	$ 32

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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