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Marketable Securities - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 28, 2013 | Nov. 30, 2017 | Mar. 31, 2018 | Dec. 31, 2017
Equity [Line Items] |  |  |  |
Marketable securities sold at market value |  |  | $ 52,200,000 |
Gain (loss) on marketable securities |  |  | 2,400,000 |
Marketable securities, unrealized gain (loss) on mark to market securities |  |  | 796,000 |
Marketable securities |  |  | 8,622,000 | $ 57,623,000
Other Income, Net [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Marketable securities, unrealized gain (loss) on mark to market securities |  |  | $ (800,000) |
NASDAQ [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Period for earn-out receivable under common stock transaction |  |  |  | 10 years
Earn-out shares received under common stock transaction |  |  |  | 992,247
Number of shares sold in transaction |  | 242,247 | 650,000 |
Remaining number of earn-out shares received under common stock transaction |  |  | 100,000 |
NASDAQ [Member] | BGC Partners, Inc. [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Period for earn-out receivable under common stock transaction | 15 years |  |  |
Maximum [Member] | NASDAQ [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Remaining earn-out receivable under common stock transaction |  |  |  | 9,922,470
Maximum [Member] | NASDAQ [Member] | BGC Partners, Inc. [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Earn-out shares receivable under common stock transaction | 14,883,705 |  |  |
Minimum [Member] | NASDAQ [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Gross revenue on earn-out receivable per year under common stock transaction |  |  |  | $ 25,000,000
Minimum [Member] | NASDAQ [Member] | BGC Partners, Inc. [Member] |  |  |  |
Equity [Line Items] |  |  |  |
Gross revenue on earn-out receivable per year under common stock transaction | $ 25,000,000 |  |  |

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- Definition Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563 + Details
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- Definition The aggregate net change in the difference between the fair value and the carrying value, or in the comparative fair values, of marketable securities categorized as trading held at each balance sheet date, that was included in earnings for the period, which may have arisen from (a) securities classified as trading, (b) the unrealized holding gain (loss) on held-to-maturity securities transferred to the trading security category, and (c) the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) on available-for-sale securities transferred to trading securities 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563 + Details
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