Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investments [Line Items]
Equity method investments	$ 35,600,000		$ 35,600,000		$ 35,200,000
Gains (losses) on equity method investments	1,530,000	$ 1,327,000	3,051,000	$ 4,962,000
Investments	35,969,000		35,969,000		35,403,000
GFI Group Inc. [Member]
Investments [Line Items]
Consolidated VIE, total assets	6,000,000.0		6,000,000.0		4,900,000
Consolidated VIE, total liabilities	1,600,000		1,600,000		600,000
Maximum exposure to loss	2,200,000		2,200,000		2,100,000
ASU 2016-01 [Member]
Investments [Line Items]
Investments	300,000		300,000		$ 200,000
Gains (losses) on equity method investments	0		0
Impairment charges, alternative measurements	0		0
Unrealized gains to reflect observable transactions	$ 0	$ 17,800,000	$ 20,400,000	$ 38,300,000