Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 08, 2017	Dec. 31, 2016
Investment [Line Items]
Gain on equity method investments	$ 2,100,000	$ 800,000	$ 4,000,000	$ 2,500,000
Equity method investments	134,905,000		134,905,000			$ 30,844,000
Gains (losses) on equity method investments	2,147,000	$ 796,000	3,986,000	$ 2,547,000
Cost method investments	3,088,000		3,088,000			$ 2,595,000
GFI Group Inc. [Member]
Investment [Line Items]
Consolidated VIE, total assets	6,300,000		6,300,000
Consolidated VIE, total liabilities	2,000,000		2,000,000
Maximum Exposure to Loss	2,600,000		2,600,000
CF Real Estate Finance Holdings, L.P. [Member]
Investment [Line Items]
Equity method investments					$ 100,000,000
Gains (losses) on equity method investments	$ 900,000		$ 900,000