Fair Value - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Total Financial instruments owned, at fair value	$ 2,444,400	$ 2,707,371
Financial instrument assets	2,517,735	2,712,820
Liabilities
Financial instruments sold, not yet purchased, at fair value	2,113,404	2,285,707
Total liabilities held at fair value	2,113,404	2,285,707
Other Assets and Investments
Assets
Financial instrument assets	5,800
KCG Hotspot
Liabilities
Receivable from BATS	64,200
KCG Hotspot | Other Assets and Investments
Liabilities
Receivable from BATS	64,200
Level 1
Assets
Total Financial instruments owned, at fair value	2,443,955	2,707,311
Financial instrument assets	2,445,769	2,711,746
Liabilities
Total liabilities held at fair value	2,113,404	2,285,707
Level 2
Assets
Total Financial instruments owned, at fair value	445	60
Financial instrument assets	66,177	1,074
Liabilities
Total liabilities held at fair value	0	0
Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Financial instrument assets	5,789	0
Liabilities
Total liabilities held at fair value	0	0
Equities
Assets
Total Financial instruments owned, at fair value	2,129,208	2,479,910
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,856,171	2,069,342
Securities netted by respective long and short positions	856,400	743,100
Equities | Level 1
Assets
Total Financial instruments owned, at fair value	2,129,208	2,479,910
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,856,171	2,069,342
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
Listed options
Assets
Total Financial instruments owned, at fair value	178,360	144,586
Liabilities
Financial instruments sold, not yet purchased, at fair value	151,893	115,362
Listed options | Level 1
Assets
Total Financial instruments owned, at fair value	178,360	144,586
Liabilities
Financial instruments sold, not yet purchased, at fair value	151,893	115,362
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
U.S. government obligations
Liabilities
Financial instruments sold, not yet purchased, at fair value	21,056	18,953
U.S. government obligations | Level 1
Liabilities
Financial instruments sold, not yet purchased, at fair value	21,056	18,953
U.S. government obligations | Level 2
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
U.S. government obligations | Level 3
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	41,706	22,983
Liabilities
Securities netted by respective long and short positions	100
U.S. government and Non-U.S. government obligations | Level 1
Assets
Total Financial instruments owned, at fair value	41,706	22,983
U.S. government and Non-U.S. government obligations | Level 2
Assets
Total Financial instruments owned, 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
Corporate debt
Assets
Total Financial instruments owned, at fair value	94,681	59,832
Liabilities
Financial instruments sold, not yet purchased, at fair value	84,284	82,050
Securities netted by respective long and short positions		300
Corporate debt | Level 1
Assets
Total Financial instruments owned, at fair value	94,681	59,832
Liabilities
Financial instruments sold, not yet purchased, at fair value	84,284	82,050
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
Foreign currency forward contracts
Assets
Total Financial instruments owned, at fair value	445	60
Foreign currency forward contracts | Level 1
Assets
Total Financial instruments owned, at fair value	0	0
Foreign currency forward contracts | Level 2
Assets
Total Financial instruments owned, at fair value	445	60
Foreign currency forward contracts | Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Investment in CME Group
Assets
Financial instrument assets	1,814	4,435
Investment in CME Group | Level 1
Assets
Financial instrument assets	1,814	4,435
Investment in CME Group | Level 2
Assets
Financial instrument assets	0	0
Investment in CME Group | Level 3
Assets
Financial instrument assets	0	0
Other investments
Assets
Financial instrument assets	71,521	1,014
Other investments | Other Assets and Investments
Assets
Financial instrument assets	5,800
Liabilities
Deferred compensation investments	1,500
Other investments | KCG Hotspot | Other Assets and Investments
Liabilities
Receivable from BATS	64,200
Other investments | Level 1
Assets
Financial instrument assets	0	0
Other investments | Level 2
Assets
Financial instrument assets	65,732	1,014
Other investments | Level 3
Assets
Financial instrument assets	$ 5,789	$ 0