Acquisitions - Additional Information (Detail) - USD ($)					3 Months Ended	12 Months Ended
	Oct. 06, 2015	Aug. 24, 2015	Apr. 28, 2015	Feb. 26, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Number of shares owned prior to Acquisition				17,100,000
Stock issued during period, value, new issues						$ 1,108,000	$ 361,000	$ 6,009,000
Additional goodwill recognized						811,766,000	392,570,000	163,339,000
Total revenues						2,575,437,000	1,787,490,000	1,744,981,000
Impairment of goodwill						121,600,000
Aggregate professional fees						62,768,000	51,823,000	$ 51,384,000
Gain on Acquisition						35,994,000
Business acquisition, contingent cash consideration						$ 22,800,000	$ 34,000,000
Class A Common Stock [Member]
Business Acquisition [Line Items]
Common stock, par value						$ 0.01	$ 0.01
Stock issued during period, shares, new issues						129,151	47,896	1,053,842
Stock issued during period, value, new issues						$ 1,000		$ 11,000
Business acquisition, equity interest issued, number of shares						2,000,000
Limited Partnership Units [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, number of shares						2,000,000
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			67.00%	56.00%		100.00%
Payment date for tendered shares						Mar. 04, 2015
Payment for tendered shares aggregate amount				$ 331,100,000
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			$ 250,000,000
Senior Notes maturity date						Jun. 19, 2018
Debt instrument, description of reference rate						LIBOR
Debt instrument, basis point			2.00%
Additional goodwill recognized				453,800,000
Total revenues						$ 551,500,000
Settlement fund		$ 10,750,000
Breakage costs						24,700,000
Severance and compensation restructuring charges						22,200,000
Aggregate professional fees						24,900,000
Gain on Acquisition					$ 29,000,000	$ 29,000,000
Business acquisition, equity interest issued, number of shares						23,500,000
Business acquisition, equity interest issued, value				$ 75,100,000
GFI Group Inc. [Member] | 5.375% JPI Note [Member]
Business Acquisition [Line Items]
Note advanced amount	$ 10,750,000
Note advanced interest rate	5.375%
Note advanced, number of shares pledged as collateral	2,000,000
GFI Group Inc. [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Common stock, par value				$ 0.01
Financial Services [Member]
Business Acquisition [Line Items]
Additional goodwill recognized				$ 453,800,000		$ 418,929,000	$ 134,898,000	85,163,000
Total consideration transferred							50,500,000
Business acquisition, contingent cash consideration							$ 1,700,000
Financial Services [Member] | Class A Common Stock [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, number of shares							300,000
Business acquisition, equity interest issued, fair value							$ 2,700,000
Financial Services [Member] | Limited Partnership Units [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, number of shares							1,300,000
Business acquisition, equity interest issued, fair value							$ 6,200,000
Financial Services [Member] | Heat Energy Group, LLC [Member]
Business Acquisition [Line Items]
Business acquisition date						Feb. 14, 2014
Financial Services [Member] | Remate Lince [Member]
Business Acquisition [Line Items]
Business acquisition date						May 09, 2014
Financial Services [Member] | Heat Energy Group, LLC, Remate Lince and RP Martin Group [Member]
Business Acquisition [Line Items]
Additional goodwill recognized							42,600,000
Financial Services [Member] | R.P. Martin Group [Member]
Business Acquisition [Line Items]
Business acquisition date						Dec. 12, 2014
Real Estate Services [Member]
Business Acquisition [Line Items]
Additional goodwill recognized						$ 392,837,000	257,672,000	$ 78,176,000
Total consideration transferred						143,600,000	201,900,000
Business acquisition, contingent cash consideration						$ 22,800,000	$ 32,300,000
Real Estate Services [Member] | Limited Partnership Units [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, number of shares						1,800,000	3,600,000
Business acquisition, equity interest issued, value						$ 15,200,000	$ 28,000,000
Real Estate Services [Member] | Class A Common Stock [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, value						$ 4,100,000	$ 13,100,000
Real Estate Services [Member] | Restricted Shares [Member] | Class A Common Stock [Member]
Business Acquisition [Line Items]
Business acquisition, equity interest issued, number of shares						500,000	1,700,000
Real Estate Services [Member] | Apartment Realty Advisors ("ARA") [Member]
Business Acquisition [Line Items]
Acquisition date						2014-12
Real Estate Services [Member] | Cornish And Carey Commercial [Member]
Business Acquisition [Line Items]
Business acquisition date						Aug. 13, 2014
Real Estate Services [Member] | Cornish & Carey Commercial and Apartment Realty Advisors [Member]
Business Acquisition [Line Items]
Additional goodwill recognized						$ 135,200,000	$ 179,500,000