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Investments - Additional Information (Detail) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Investment [Line Items] |  |  |
Equity method investments | $ 36,400,000 |  | $ 35,200,000
Gains (losses) on equity method investments | 783,000 | $ 2,624,000 |
Investments | 36,708,000 |  | 35,403,000
GFI Group Inc. [Member] |  |  |
Investment [Line Items] |  |  |
Consolidated VIE, total assets | 5,000,000 |  | 4,900,000
Consolidated VIE, total liabilities | 800,000 |  | 600,000
Maximum exposure to loss | 2,200,000 |  | 2,100,000
ASU 2016-01 [Member] |  |  |
Investment [Line Items] |  |  |
Investments | 300,000 |  | $ 200,000
Gains (losses) on equity method investments | 0 |  |
Impairment charges, alternative measurements | 0 |  |
ASU 2016-01 [Member] | LCH.Clearnet Group Limited [Member] |  |  |
Investment [Line Items] |  |  |
Unrealized gains to reflect observable transactions | $ 20,400,000 | $ 20,600,000 |

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