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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Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

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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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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 ($)	3 Months Ended		9 Months Ended
Investments - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 08, 2017
Investment [Line Items]
Equity method investments	$ 136,100,000		$ 136,100,000		$ 135,600,000
Gains (losses) on equity method investments	1,400,000	$ 2,100,000	10,000,000	$ 4,000,000
Gains (losses) on equity method investments	1,344,000	2,147,000	9,999,000	3,986,000
Cost method investments					6,200,000
Gain (loss) relating to cost method investments		$ 0		$ 0
Investments	164,892,000		164,892,000		141,788,000
GFI Group Inc. [Member]
Investment [Line Items]
Consolidated VIE, total assets	7,300,000		7,300,000		5,900,000
Consolidated VIE, total liabilities	900,000		900,000		1,600,000
Maximum Exposure to Loss	3,500,000		3,500,000		$ 2,400,000
ASU 2016-01 [Member]
Investment [Line Items]
Gains (losses) on equity method investments	0		0
Investments	28,800,000		28,800,000
Impairment charges, alternative measurements	0		0
ASU 2016-01 [Member] \| LCH.Clearnet Group Limited [Member]
Investment [Line Items]
Unrealized gains to reflect observable transactions	18,200,000		38,700,000
CF Real Estate Finance Holdings, L.P. [Member]
Investment [Line Items]
Equity method investments						$ 100,000,000
Gains (losses) on equity method investments	$ 0		$ 5,000,000