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Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale securities, Less Than 12 Months, Market Value | $ 31,723 | $ 55,440
Available for sale securities, Less Than 12 Months, Unrealized Losses | 149 | 136
Available for sale securities, 12 Months or Greater, Market Value | 22,610 |
Available for sale securities, 12 Months or Greater, Unrealized Losses | 39 |
Federal Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale securities, Less Than 12 Months, Market Value |  | 1,793
Available for sale securities, Less Than 12 Months, Unrealized Losses |  | 7
Available for sale securities, 12 Months or Greater, Market Value | 1,793 |
Available for sale securities, 12 Months or Greater, Unrealized Losses | 7 |
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale securities, Less Than 12 Months, Market Value | 31,723 | 53,647
Available for sale securities, Less Than 12 Months, Unrealized Losses | 149 | $ 129
Available for sale securities, 12 Months or Greater, Market Value | 20,817 |
Available for sale securities, 12 Months or Greater, Unrealized Losses | $ 32 |

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- Definition 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. + 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)(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 + Details
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- Definition 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 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 + Details
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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 + Details
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