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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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

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The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Investments - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
Investments - Additional Information (Detail) - USD ($)	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investment [Line Items]
Equity method investments	$ 36,400,000		$ 36,400,000		$ 35,200,000
Gains (losses) on equity method investments	738,000	$ 1,011,000	1,521,000	$ 3,635,000
Investments	36,661,000		36,661,000		35,403,000
GFI Group Inc. [Member]
Investment [Line Items]
Consolidated VIE, total assets	4,800,000		4,800,000		4,900,000
Consolidated VIE, total liabilities	800,000		800,000		600,000
Maximum exposure to loss	2,000,000.0		2,000,000.0		2,100,000
ASU 2016-01 [Member]
Investment [Line Items]
Investments	300,000		300,000		$ 200,000
Gains (losses) on equity method investments	0		0
Impairment charges, alternative measurements	0		0
ASU 2016-01 [Member] \| LCH.Clearnet Group Limited [Member]
Investment [Line Items]
Unrealized gains to reflect observable transactions	$ 0	$ 0	$ 20,400,000	$ 20,600,000