Notes Payable, Collateralized and Short-Term Borrowings - Additional Information (Detail) - USD ($)						3 Months Ended		12 Months Ended
	Jul. 15, 2016	Jul. 13, 2016	Feb. 25, 2016	Oct. 01, 2015	Apr. 28, 2015	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	May 27, 2016	Jan. 12, 2016	Jun. 23, 2015	Mar. 13, 2015	Dec. 09, 2014	Jun. 26, 2012	Jul. 29, 2011
Debt Instrument [Line Items]
Redemption price as percentage of principal amount						100.00%
Carrying amount of debt component						$ 963,386,000		$ 965,767,000
Outstanding balance of the secured loan arrangements						14,500,000
Interest expense						14,821,000	$ 13,458,000
Cantor [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest expense						$ 0	0
Maximum revolving credit				$ 150,000,000
Credit agreement maturity date						Aug. 10, 2017
Debt instrument, description of reference rate						LIBOR
Debt instrument, basis point				3.25%
Line of credit facility, borrowing amount				$ 100,000,000
Line of credit facility, description						The Cantor Loan bears interest at the rate of LIBOR plus 3.25% and may be adjusted based on Cantor’s short-term borrowing rate then in effect, plus 1%.
Cantor [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Principal amount of notes											$ 150,000,000
Secured Loan Arrangements [Member]
Debt Instrument [Line Items]
Senior Notes maturity date						Mar. 13, 2019
Stated interest rate												3.70%
Debt instrument gross amount												$ 28,200,000
Deferred financing costs						$ 100,000
Value of the fixed assets pledged						2,400,000
Interest expense						$ 200,000	200,000
GFI Group Inc. [Member]
Debt Instrument [Line Items]
Percentage of outstanding common stock owned by parent										100.00%
Class A Common Stock [Member]
Debt Instrument [Line Items]
Convertible Notes, shares								6,909
Stock issued during period, shares, new issues						9,942		2,004,533
4.50% Convertible Notes [Member]
Debt Instrument [Line Items]
Principal amount of notes															$ 160,000,000
Senior Notes maturity date	Jul. 15, 2016					Jul. 15, 2016
Interest expense recorded on Senior Notes						$ 0	3,000,000
Stated interest rate	4.50%	4.50%				4.50%									4.50%
Capped call transactions expiration date		Jul. 13, 2016				Jul. 13, 2016
Repayments of convertible notes	$ 159,900,000
Principal amount of notes		$ 68,000
4.50% Convertible Notes [Member] | Class A Common Stock [Member]
Debt Instrument [Line Items]
Convertible Notes, shares		6,909
8.125% Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount of notes														$ 112,500,000
Interest expense recorded on Senior Notes						$ 2,300,000	2,300,000
Stated interest rate						8.125%		8.125%
Debt maturity year						2042
Earliest call date						Jun. 26, 2017
Carrying amount of debt component						$ 109,302,000		$ 109,271,000						108,700,000
Debt issuance costs														$ 3,800,000
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount of notes													$ 300,000,000
Senior Notes maturity date						Dec. 09, 2019
Interest expense recorded on Senior Notes						$ 4,300,000	4,300,000
Stated interest rate						5.375%		5.375%					5.375%
Debt maturity year						2019
Redemption price as percentage of principal amount						101.00%
Carrying amount of debt component						$ 297,328,000		$ 297,083,000					$ 295,100,000
Debt issuance costs													$ 4,900,000
Debt instrument, periodic interest payment description						payable in cash on June 9 and December 9 of each year, commencing June 9, 2015.
8.375% Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount of notes						$ 240,000,000
Interest expense recorded on Senior Notes						$ 5,000,000	5,100,000
Stated interest rate						8.375%		8.375%
Carrying amount of debt component						$ 245,875,000		$ 246,988,000
Debt instrument, periodic interest payment description						semi-annually
Debt maturity date						2018-07
Senior notes, carrying value						$ 245,900,000
Debt instrument penalty interest						2.00%
8.375% Senior Notes [Member] | GFI Group Inc. [Member]
Debt Instrument [Line Items]
Stated interest rate						8.375%
Stock issued during period, shares, new issues					43,000,000
Percentage of outstanding common stock owned by parent					67.00%
Debt instrument penalty interest						0.25%
5.125% Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount of notes									$ 300,000,000
Senior Notes maturity date						May 27, 2021
Interest expense recorded on Senior Notes						$ 4,000,000	0
Stated interest rate						5.125%		5.125%	5.125%
Debt maturity year						2021
Redemption price as percentage of principal amount						101.00%
Carrying amount of debt component						$ 296,406,000		$ 296,215,000	$ 295,800,000
Debt issuance costs									$ 4,200,000
Debt instrument, periodic interest payment description						payable in cash on May 27 and November 27 of each year, commencing November 27, 2016.
Unsecured Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit						$ 25,000,000
Credit agreement maturity date						Mar. 24, 2016
Debt instrument, description of reference rate						LIBOR
Debt instrument, basis point						2.00%
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest expense recorded on Senior Notes						$ 200,000	$ 100,000
Maximum revolving credit			$ 150,000,000
Credit agreement maturity date			Feb. 25, 2018
Debt instrument, description of reference rate						LIBOR
Line of credit facility, description						Borrowings under this facility bear interest at either LIBOR or a defined base rate plus an additional margin which ranges from 50 basis points to 250 basis points depending on the Company’s debt rating as determined by S&P and Fitch and whether such loan is a LIBOR loan or a base rate loan.
Maximum revolving credit, option to increase			$ 200,000,000
Maximum revolving credit terminated			$ 25,000,000
Line of credit facility, outstanding amount						$ 0
Line of credit facility, terminated outstanding amount						$ 0
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			0.50%
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			2.50%