Marketable Securities - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 28, 2013	Nov. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Equity [Line Items]
Marketable securities sold at market value			$ 52,200,000
Gain (loss) on marketable securities			2,400,000
Marketable securities, unrealized gain (loss) on mark to market securities			796,000
Marketable securities			8,622,000	$ 57,623,000
Other Income, Net [Member]
Equity [Line Items]
Marketable securities, unrealized gain (loss) on mark to market securities			$ (800,000)
NASDAQ [Member]
Equity [Line Items]
Period for earn-out receivable under common stock transaction				10 years
Earn-out shares received under common stock transaction				992,247
Number of shares sold in transaction		242,247	650,000
Remaining number of earn-out shares received under common stock transaction			100,000
NASDAQ [Member] | BGC Partners, Inc. [Member]
Equity [Line Items]
Period for earn-out receivable under common stock transaction	15 years
Maximum [Member] | NASDAQ [Member]
Equity [Line Items]
Remaining earn-out receivable under common stock transaction				9,922,470
Maximum [Member] | NASDAQ [Member] | BGC Partners, Inc. [Member]
Equity [Line Items]
Earn-out shares receivable under common stock transaction	14,883,705
Minimum [Member] | NASDAQ [Member]
Equity [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction				$ 25,000,000
Minimum [Member] | NASDAQ [Member] | BGC Partners, Inc. [Member]
Equity [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000