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Investments - Additional Information (Detail) - USD ($)	3 Months Ended
Investments - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Sep. 08, 2017
Investment [Line Items]
Equity method investments	$ 139,700	$ 135,600
Gains (losses) on equity method investments	5,800,000	200,000
Gains (losses) on equity method investments	5,801,000	237,000
Cost method investments			$ 6,200,000
Gain (loss) relating to cost method investments		$ 0
Investments	153,452,000		141,788,000
GFI Group Inc. [Member]
Investment [Line Items]
Consolidated VIE, total assets	7,400,000
Consolidated VIE, total liabilities	1,900,000		1,600,000
Maximum Exposure to Loss	3,500,000		$ 2,400,000
ASU 2016-01 [Member]
Investment [Line Items]
Gains (losses) on equity method investments	0
Investments	13,800,000
Impairment charges, alternative measurements	0
ASU 2016-01 [Member] \| LCH.Clearnet Group Limited [Member]
Investment [Line Items]
Impairment charges, alternative measurements	0
Unrealized gain to reflect observable transaction	20,600,000
CF Real Estate Finance Holdings, L.P. [Member]
Investment [Line Items]
Equity method investments				$ 100,000,000
Gains (losses) on equity method investments	$ 3,200,000