Fair Value - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Total Financial instruments owned, at fair value	$ 2,320,442	$ 2,539,861
Total assets held at fair value	2,382,035	2,614,729
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,907,975	2,046,140
Total liabilities held at fair value	1,907,975	2,046,140
Level 1
Assets
Total Financial instruments owned, at fair value	2,320,442	2,539,831
Total assets held at fair value	2,322,821	2,549,029
Liabilities
Total liabilities held at fair value	1,907,377	2,046,140
Level 2
Assets
Total Financial instruments owned, at fair value	0	30
Total assets held at fair value	56,376	62,854
Liabilities
Total liabilities held at fair value	598	0
Investments	2,200	2,300
Level 3
Assets
Total Financial instruments owned, at fair value	0	0
Total assets held at fair value	2,838	2,846
Liabilities
Total liabilities held at fair value	0	0
Equities
Assets
Total Financial instruments owned, at fair value	2,179,211	2,343,033
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,694,374	1,821,957
Equities | Level 1
Assets
Total Financial instruments owned, at fair value	2,179,211	2,343,033
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,694,374	1,821,957
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	91,314	127,237
Liabilities
Financial instruments sold, not yet purchased, at fair value	85,926	123,561
Corporate debt | Level 1
Assets
Total Financial instruments owned, at fair value	91,314	127,237
Liabilities
Financial instruments sold, not yet purchased, at fair value	85,926	123,561
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
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	37,618	50,461
Liabilities
Financial instruments sold, not yet purchased, at fair value	91,242	87,661
U.S. government and Non-U.S. government obligations | Level 1
Assets
Total Financial instruments owned, at fair value	37,618	50,461
Liabilities
Financial instruments sold, not yet purchased, at fair value	91,242	87,661
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
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Listed options
Assets
Total Financial instruments owned, at fair value	12,299	19,100
Liabilities
Financial instruments sold, not yet purchased, at fair value	35,835	12,961
Listed options | Level 1
Assets
Total Financial instruments owned, at fair value	12,299	19,100
Liabilities
Financial instruments sold, not yet purchased, at fair value	35,835	12,961
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		30
Liabilities
Financial instruments sold, not yet purchased, at fair value	598
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		30
Liabilities
Financial instruments sold, not yet purchased, at fair value	598
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
Investments
Assets
Total assets held at fair value	4,613	11,484
Investments | Level 1
Assets
Total assets held at fair value	2,379	9,198
Liabilities
Investments	2,400
Investments | Level 2
Assets
Total assets held at fair value	2,234	2,286
Investments | Level 3
Assets
Total assets held at fair value	0	0
CME Group and BATS | Level 1
Liabilities
Investments		9,200
Other
Assets
Total assets held at fair value	56,980	63,384
Other | Other Assets and Investments
Assets
Total assets held at fair value	2,800	2,800
Other | KCG Hotspot | Other Assets and Investments
Liabilities
Receivable from BATS	54,100	60,500
Other | Level 1
Assets
Total assets held at fair value	0	0
Other | Level 2
Assets
Total assets held at fair value	54,142	60,538
Other | Level 3
Assets
Total assets held at fair value	$ 2,838	$ 2,846