Notes Payable, Collateralized and Short-Term Borrowings - Additional Information (Detail) - USD ($)					3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 15, 2016	Jul. 13, 2016	Feb. 25, 2016	Apr. 28, 2015	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	May 31, 2017	May 27, 2016	Jan. 12, 2016	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					$ 1,140,887,000		$ 1,140,887,000		$ 965,767,000
Interest expense					16,676,000	$ 14,624,000	$ 31,497,000	$ 28,082,000
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							33,616		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
Interest expense recorded					$ 0	$ 3,000,000	$ 0	$ 6,100,000
Stated interest rate	4.50%	4.50%			4.50%	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					$ 2,300,000	$ 2,300,000	$ 4,600,000	$ 4,600,000
Stated interest rate					8.125%		8.125%		8.125%
Debt maturity year							2042
Earliest call date							Jun. 26, 2017
Carrying amount of debt component					$ 109,334,000		$ 109,334,000		$ 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					$ 4,300,000	4,300,000	$ 8,600,000	8,600,000
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					$ 297,573,000		$ 297,573,000		$ 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		$ 240,000,000
Interest expense recorded					$ 5,000,000	5,000,000	$ 10,000,000	10,100,000
Stated interest rate					8.375%		8.375%		8.375%
Carrying amount of debt component					$ 244,750,000		$ 244,750,000		$ 246,988,000
Debt instrument, periodic interest payment description							semi-annually
Debt maturity date							2018-07
Senior notes, carrying value					$ 244,700,000		$ 244,700,000
Debt instrument penalty interest							2.00%
8.375% Senior Notes [Member] | GFI Group Inc. [Member]
Debt Instrument [Line Items]
Stated interest rate					8.375%		8.375%
Debt instrument penalty interest							0.25%
Stock issued during period, shares, new issues				43,000,000
Percentage of outstanding common stock owned by parent				67.00%
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					$ 4,000,000	1,600,000	$ 8,100,000	1,600,000
Stated interest rate					5.125%		5.125%		5.125%		5.125%
Debt maturity year							2021
Redemption price as percentage of principal amount							101.00%
Carrying amount of debt component					$ 296,600,000		$ 296,600,000		$ 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
3.70% [Member]
Debt Instrument [Line Items]
Outstanding balance of the secured loan arrangements					12,700,000		$ 12,700,000
3.70% [Member] | 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		$ 100,000
Value of the fixed assets pledged					1,500,000		1,500,000
Interest expense					100,000	200,000	300,000	400,000
3.44% [Member]
Debt Instrument [Line Items]
Outstanding balance of the secured loan arrangements					29,900,000		$ 29,900,000
3.44% [Member] | Secured Loan Arrangements [Member]
Debt Instrument [Line Items]
Senior Notes maturity date							May 31, 2021
Stated interest rate										3.44%
Debt instrument gross amount										$ 29,900,000
Value of the fixed assets pledged					25,800,000		$ 25,800,000
Interest expense					100,000		100,000
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest expense recorded					1,000,000	$ 200,000	$ 1,200,000	$ 300,000
Maximum revolving credit			$ 150,000,000
Maximum revolving credit, option to increase			$ 200,000,000
Credit agreement maturity date			Feb. 25, 2018
Maximum revolving credit terminated			$ 25,000,000
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.
Line of credit facility, outstanding amount					$ 150,000,000		$ 150,000,000		$ 0
Debt instrument, description of reference rate							LIBOR
Current interest rate					3.22%		3.22%
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%