Marketable Securities - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 28, 2013	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Equity [Line Items]
Gross proceeds from sale of marketable securities				$ 32,606,000	$ 51,433,000
Gain on sale of marketable securities		$ 2,200,000	$ 0.0	4,056,000	1,494,000
Marketable securities, unrealized gain (loss) on mark to market securities				4,445,000	$ 897,000
Marketable securities		$ 123,417,000		$ 123,417,000		$ 48,942,000
Nasdaq [Member]
Equity [Line Items]
Number of shares sold in transaction				350,000	650,000
Remaining number of earn-out shares received under common stock transaction		250,000		250,000
Certain Assets of Nasdaq [Member]
Equity [Line Items]
Period for earn-out receivable under common stock transaction				8 years
Amount recognized in connection with the earn-out including other income (loss)						992,247
Other Income, Net [Member]
Equity [Line Items]
Gain on earn-out, common stock transaction		$ 98,600	85,100
Marketable securities, unrealized gain (loss) on mark to market securities		$ (700,000)	$ (600,000)	$ 4,400,000	$ 900,000
Certain Assets of Nasdaq [Member] | BGC Partners Inc [Member]
Equity [Line Items]
Period for earn-out receivable under common stock transaction	15 years
Maximum [Member] | Certain Assets of Nasdaq [Member]
Equity [Line Items]
Remaining earn-out receivable under common stock transaction				7,937,976
Maximum [Member] | Certain Assets of Nasdaq [Member] | NEWMARK Group Inc Parent [Member]
Equity [Line Items]
Earn-out shares receivable under common stock transaction	14,883,705
Minimum [Member] | Certain Assets of Nasdaq [Member]
Equity [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction				$ 25,000,000.0
Minimum [Member] | Certain Assets of Nasdaq [Member] | NEWMARK Group Inc Parent [Member]
Equity [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0