Acquisitions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Jan. 12, 2016	Apr. 28, 2015	Feb. 26, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 04, 2016
Business Acquisition [Line Items]
Number of shares owned prior to Acquisition			17,100,000
Stock issued during period, value, new issues				$ 17,778	$ 1,108	$ 361
Cash paid in connection with closing of Back-End Mergers				36,800
Additional goodwill recognized				863,690	811,766	392,570
Total revenues				2,612,623	2,580,439	1,789,579
Aggregate professional fees				60,920	66,382	52,598
Gain on Acquisition				13,410	35,994
Business acquisition, contingent cash consideration				18,500	22,800	$ 34,000
Additional goodwill recognized				53,367	$ 581,518
Earn-out payment reduced				17,300
Cash returned				1,000
Proceeds from reduction				$ 18,300
Partnership Units [Member]
Business Acquisition [Line Items]
Partnership units returned				1,600,000
Acquisition date fair value				$ 14,900
Class A Common Stock [Member]
Business Acquisition [Line Items]
Common stock, par value				$ 0.01	$ 0.01
Stock issued during period, shares, new issues				2,004,533	129,151	47,896
Stock issued during period, value, new issues				$ 20	$ 1
Business acquisition, equity interest issued, number of shares				1,900,000
GFI Group Inc. [Member]
Business Acquisition [Line Items]
Business acquisition date				Feb. 26, 2015
Common stock, par value			$ 6.10
Tendered shares purchased			54,300,000
Percentage of outstanding common stock owned by parent	100.00%	67.00%	56.00%	100.00%
Payment date for tendered shares				Mar. 04, 2015
Payment for tendered shares aggregate amount			$ 331,100
Number of shares owned prior to Acquisition				17,100,000	17,100,000
Stock issued during period, shares, new issues		43,000,000
Closing price per share		$ 5.81
Stock issued during period, value, new issues		$ 250,000
Senior Notes maturity date				Jun. 19, 2018
Debt instrument, description of reference rate				LIBOR
Debt instrument, basis point		2.00%
Business acquisition, equity interest issued, number of shares				23,500,000
Cash paid in connection with closing of Back-End Mergers	$ 111,200		331,100	$ 89,900
Additional goodwill recognized			450,000
Total revenues				544,500	$ 551,500
Breakage costs				24,700
Severance and compensation restructuring charges				22,200
Aggregate professional fees				24,900
Gain on Acquisition				29,000	29,000
Ownership percentage acquired	100.00%
Acquisition date fair value			$ 75,100
GFI Group Inc. [Member] | Class A Common Stock [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, number of shares	23,500,000
GFI Group Inc. [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Common stock, par value			$ 0.01
Lucera [Member]
Business Acquisition [Line Items]
Ownership percentage acquired							80.00%
Other Acquisitions [Member]
Business Acquisition [Line Items]
Total consideration transferred				139,000	143,600
Business acquisition, contingent cash consideration				28,100	42,000
Additional goodwill recognized				$ 53,400	135,200
Perimeter Market Inc. [Member]
Business Acquisition [Line Items]
Business acquisition date				Sep. 23, 2016
Walchle Lear Multifamily Advisors [Member]
Business Acquisition [Line Items]
Business acquisition date				Dec. 14, 2016
Sunrise Brokers Group [Member]
Business Acquisition [Line Items]
Business acquisition date				Dec. 15, 2016
Financial Services [Member]
Business Acquisition [Line Items]
Additional goodwill recognized			$ 450,000	$ 451,035	418,929	$ 134,898
Additional goodwill recognized				$ 36,281	$ 453,833