Fair Value - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Total Financial instruments owned, at fair value	$ 2,904,953	$ 2,444,400
Total assets held at fair value	2,975,847	2,517,735
Liabilities
Financial instruments sold, not yet purchased, at fair value	2,222,788	2,113,404
Total liabilities held at fair value	2,222,788	2,113,404
KCG Hotspot | Other Assets and Investments
Liabilities
Receivable from BATS	59,900
Level 1
Assets
Total Financial instruments owned, at fair value	2,904,953	2,443,955
Total assets held at fair value	2,907,046	2,445,769
Liabilities
Total liabilities held at fair value	2,222,486	2,113,404
Level 2
Assets
Total Financial instruments owned, at fair value	0	445
Total assets held at fair value	62,151	66,177
Liabilities
Total liabilities held at fair value	302	0
Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Total assets held at fair value	6,650	5,789
Liabilities
Total liabilities held at fair value	0	0
Equities
Assets
Total Financial instruments owned, at fair value	2,641,499	2,129,208
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,927,864	1,856,171
Equities | Level 1
Assets
Total Financial instruments owned, at fair value	2,641,499	2,129,208
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,927,864	1,856,171
Equities | Level 2
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Equities | Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Corporate debt
Assets
Total Financial instruments owned, at fair value	179,152	94,681
Liabilities
Financial instruments sold, not yet purchased, at fair value	173,678	84,284
Corporate debt | Level 1
Assets
Total Financial instruments owned, at fair value	179,152	94,681
Liabilities
Financial instruments sold, not yet purchased, at fair value	173,678	84,284
Corporate debt | Level 2
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Corporate debt | Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
U.S. government and Non-U.S. government obligations
Assets
Total Financial instruments owned, at fair value	72,112	41,706
Liabilities
Financial instruments sold, not yet purchased, at fair value	104,625	21,056
U.S. government and Non-U.S. government obligations | Level 1
Assets
Total Financial instruments owned, at fair value	72,112	41,706
Liabilities
Financial instruments sold, not yet purchased, at fair value	104,625	21,056
U.S. government and Non-U.S. government obligations | Level 2
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
U.S. government and Non-U.S. government obligations | Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Listed options
Assets
Total Financial instruments owned, at fair value	12,190	178,360
Liabilities
Financial instruments sold, not yet purchased, at fair value	16,319	151,893
Listed options | Level 1
Assets
Total Financial instruments owned, at fair value	12,190	178,360
Liabilities
Financial instruments sold, not yet purchased, at fair value	16,319	151,893
Listed options | Level 2
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Listed options | Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Foreign currency forward contracts
Assets
Total Financial instruments owned, at fair value		445
Liabilities
Financial instruments sold, not yet purchased, at fair value	302
Foreign currency forward contracts | Level 1
Assets
Total Financial instruments owned, at fair value		0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0
Foreign currency forward contracts | Level 2
Assets
Total Financial instruments owned, at fair value		445
Liabilities
Financial instruments sold, not yet purchased, at fair value	302
Foreign currency forward contracts | Level 3
Assets
Total Financial instruments owned, at fair value		0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0
Investment in CME Group
Assets
Total assets held at fair value	2,093	1,814
Investment in CME Group | Level 1
Assets
Total assets held at fair value	2,093	1,814
Investment in CME Group | Level 2
Assets
Total assets held at fair value	0	0
Investment in CME Group | Level 3
Assets
Total assets held at fair value	0	0
Other
Assets
Total assets held at fair value	68,801	71,521
Other | Other Assets and Investments
Assets
Total assets held at fair value	6,700	5,800
Liabilities
Deferred compensation investments	2,300	1,500
Other | KCG Hotspot | Other Assets and Investments
Liabilities
Receivable from BATS	59,900	64,200
Other | Level 1
Assets
Total assets held at fair value	0	0
Other | Level 2
Assets
Total assets held at fair value	62,151	65,732
Other | Level 3
Assets
Total assets held at fair value	$ 6,650	$ 5,789