Investments - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Investment [Line Items]
Gain on equity method investments	$ 1,600,000	$ 900,000	$ 1,800,000	$ 1,800,000
Cost method investments	2,896,000		2,896,000		$ 2,595,000
GFI Group Inc. [Member]
Investment [Line Items]
Consolidated VIE, total assets	6,000,000		6,000,000
Consolidated VIE, total liabilities	1,500,000		1,500,000
Maximum Exposure to Loss	$ 2,800,000		$ 2,800,000