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Marketable Securities - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 28, 2013 | Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Equity [Line Items] |  |  |  |  |  |
Gross proceeds from sale of marketable securities |  |  |  | $ 22,204,000 | $ 51,433,000 |
Gain (loss) on marketable securities |  | $ 1,900,000 | $ 900,000 | 1,812,000 | 1,494,000 |
Marketable securities, unrealized gain (loss) on mark to market securities |  |  |  | 5,110,000 | 1,444,000 |
Marketable securities |  | 33,659,000 |  | 33,659,000 |  | $ 48,942,000
Other Income (Loss), Net [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Marketable securities, unrealized gain (loss) on mark to market securities |  | $ 1,200,000 | $ 600,000 | $ 5,100,000 | $ 1,500,000 |
Nasdaq [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Number of shares sold in transaction |  |  |  | 250,000 |  |
Remaining number of earn-out shares received under common stock transaction |  | 350,000 |  | 350,000 |  |
Certain Assets of Nasdaq [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Period for earn-out receivable under common stock transaction |  |  |  | 9 years |  |
Amount recognized in connection with the earn-out including other income (loss) |  |  |  |  |  | 992,247
Certain Assets of Nasdaq [Member] | BGC Partners Inc [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Period for earn-out receivable under common stock transaction | 15 years |  |  |  |  |
Maximum [Member] | Certain Assets of Nasdaq [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Remaining earn-out receivable under common stock transaction |  |  |  | 8,930,223 |  |
Maximum [Member] | Certain Assets of Nasdaq [Member] | NEWMARK Group Inc Parent [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Earn-out shares receivable under common stock transaction | 14,883,705 |  |  |  |  |
Minimum [Member] | Certain Assets of Nasdaq [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Gross revenue on earn-out receivable per year under common stock transaction |  |  |  | $ 25,000,000.0 |  |
Minimum [Member] | Certain Assets of Nasdaq [Member] | NEWMARK Group Inc Parent [Member] |  |  |  |  |  |
Equity [Line Items] |  |  |  |  |  |
Gross revenue on earn-out receivable per year under common stock transaction | $ 25,000,000.0 |  |  |  |  |

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