Investments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Investment [Line Items]
Gain on equity method investments	$ 200,000	$ 900,000
Cost method investments	2,897,000		$ 2,595,000
GFI Group Inc. [Member]
Investment [Line Items]
Consolidated VIE, total assets	6,200,000
Consolidated VIE, total liabilities	1,500,000
Maximum exposure to economic loss	$ 2,700,000