Long-term Debt, Collateralized and Short-term Borrowings - Additional Information (Detail)								3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 08, 2017 USD ($)	Aug. 23, 2017 USD ($)	Aug. 22, 2017 USD ($)	Jul. 15, 2016 USD ($)	Jul. 13, 2016 USD ($) shares	Feb. 25, 2016 USD ($)	Apr. 28, 2015 shares	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) shares	Aug. 23, 2017 BRL	Aug. 22, 2017 BRL	May 31, 2017 USD ($)	May 27, 2016 USD ($)	Jan. 12, 2016	Mar. 13, 2015 USD ($)	Dec. 09, 2014 USD ($)	Jun. 26, 2012 USD ($)	Jul. 29, 2011 USD ($)
Debt Instrument [Line Items]
Redemption price as percentage of principal amount										100.00%
Carrying amount of debt component								$ 1,962,120,000		$ 1,962,120,000		$ 965,767,000
Repayments of convertible notes											$ 159,932,000
Interest expense								24,425,000	$ 18,831,000	$ 69,678,000	53,770,000
Class A Common Stock [Member]
Debt Instrument [Line Items]
Stock issued during period, shares, new issues | shares										45,490		2,004,533
Convertible Notes, shares | shares												6,909
GFI Group Inc. [Member]
Debt Instrument [Line Items]
Percentage of outstanding common stock owned by parent																	100.00%
Unsecured Credit Agreement September 8, 2017 [Member] | Senior Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit	$ 400,000,000
Credit agreement maturity date										Sep. 08, 2019
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 325 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.
Debt instrument gross amount								400,000,000		$ 400,000,000
Line of credit facility, outstanding amount								396,900,000		396,900,000
Deferred financing costs								$ 3,100,000		$ 3,100,000
Current interest rate								3.49%		3.49%
Interest expense recorded								$ 1,100,000		$ 1,100,000
Unsecured Credit Agreement September 8, 2017 [Member] | Senior Revolving Credit Agreement [Member] | LIBOR or Defined Base Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point	0.50%
Unsecured Credit Agreement September 8, 2017 [Member] | Senior Revolving Credit Agreement [Member] | LIBOR or Defined Base Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point	3.25%
Unsecured Credit Agreement September 8, 2017 [Member] | Senior Term Loan Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit	$ 575,000,000
Credit agreement maturity date										Sep. 08, 2019
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 325 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.
Debt instrument gross amount								575,000,000		$ 575,000,000
Line of credit facility, outstanding amount								572,200,000		572,200,000
Deferred financing costs								$ 2,800,000		$ 2,800,000
Current interest rate								3.49%		3.49%
Interest expense recorded								$ 1,500,000		$ 1,500,000
Percentage of net cash proceedes of all material asset sales	100.00%
Unsecured Credit Agreement September 8, 2017 [Member] | Senior Term Loan Credit Agreement [Member] | LIBOR or Defined Base Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point	0.50%
Unsecured Credit Agreement September 8, 2017 [Member] | Senior Term Loan Credit Agreement [Member] | LIBOR or Defined Base Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point	3.25%
8.125% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded								$ 2,300,000	2,300,000	$ 6,900,000	6,900,000
Principal amount of notes																				$ 112,500,000
Stated interest rate								8.125%		8.125%		8.125%
Debt maturity year										2042
Earliest call date										Jun. 26, 2017
Carrying amount of debt component																				108,700,000
Debt issuance costs																				$ 3,800,000
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded								$ 4,300,000	4,300,000	$ 12,800,000	12,800,000
Principal amount of notes																			$ 300,000,000
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																			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.
Senior Notes maturity date										Dec. 09, 2019
8.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded								$ 5,000,000	5,000,000	$ 15,100,000	15,100,000
Principal amount of notes								$ 240,000,000		$ 240,000,000
Stated interest rate								8.375%		8.375%		8.375%
Debt instrument, periodic interest payment description										semi-annually
Debt maturity date										2018-07
Senior notes, carrying value								$ 243,600,000		$ 243,600,000
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 | shares							43,000,000
Percentage of outstanding common stock owned by parent							67.00%
5.125% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded								$ 4,000,000	4,000,000	$ 12,100,000	5,600,000
Principal amount of notes																$ 300,000,000
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																$ 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
Senior Notes maturity date										May 27, 2021
4.50% Convertible Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded								$ 0	$ 500,000	$ 0	$ 6,600,000
Principal amount of notes																					$ 160,000,000
Stated interest rate				4.50%	4.50%			4.50%	4.50%	4.50%	4.50%										4.50%
Senior Notes maturity date										Jul. 15, 2016
Capped call transactions expiration date					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 | shares					6,909
3.70% [Member]
Debt Instrument [Line Items]
Outstanding balance of the secured loan arrangements								$ 10,900,000		$ 10,900,000
3.70% [Member] | Secured Loan Arrangements [Member]
Debt Instrument [Line Items]
Debt instrument gross amount																		$ 28,200,000
Deferred financing costs								100,000		$ 100,000
Stated interest rate																		3.70%
Senior Notes maturity date										Mar. 13, 2019
Value of the fixed assets pledged								1,100,000		$ 1,100,000
Interest expense								100,000	$ 200,000	400,000	$ 600,000
3.44% [Member]
Debt Instrument [Line Items]
Outstanding balance of the secured loan arrangements								28,200,000		$ 28,200,000
3.44% [Member] | Secured Loan Arrangements [Member]
Debt Instrument [Line Items]
Debt instrument gross amount															$ 29,900,000
Stated interest rate															3.44%
Senior Notes maturity date										May 31, 2021
Value of the fixed assets pledged								21,600,000		$ 21,600,000
Interest expense								200,000		300,000
Unsecured Credit Agreement [Member] | Intra-Day Overdraft Credit Line [Member]
Debt Instrument [Line Items]
Maximum revolving credit		$ 15,800,000											BRL 50,000,000
Credit agreement maturity date		Dec. 14, 2017
Line of credit facility, outstanding amount								0		0
Line of credit facility, fee percentage		1.00%
Line of credit facility, fee								100,000		$ 100,000
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum revolving credit						$ 150,000,000
Credit agreement maturity date						Sep. 08, 2017
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								0		$ 0		$ 0
Interest expense recorded								1,200,000	$ 200,000	$ 2,400,000	$ 500,000
Maximum revolving credit, option to increase						$ 200,000,000
Line of credit facility, terminated outstanding amount	$ 150,000,000
Debt instrument, description of reference rate										LIBOR
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member] | LIBOR or Defined Base Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point						0.50%
Unsecured Credit Agreement [Member] | Revolving Credit Facility [Member] | LIBOR or Defined Base Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point						2.50%
Unsecured Loan Agreement [Member] | Short Term Loans [Member]
Debt Instrument [Line Items]
Maximum revolving credit			$ 6,300,000											BRL 20,000,000
Credit agreement maturity date			Feb. 19, 2018
Line of credit facility, description										Borrowings under this facility bear interest at the Brazilian Interbank offering rate plus 3.30%.
Line of credit facility, outstanding amount								6,300,000		$ 6,300,000
Current interest rate			12.92%											12.92%
Interest expense recorded								$ 100,000		$ 100,000
Debt instrument, description of reference rate										Brazilian Interbank offering rate
Unsecured Loan Agreement [Member] | Brazilian Interbank Offering Rate [Member] | Short Term Loans [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			3.30%