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

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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The difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence.

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