Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Operations (Parent Only) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Investment income and other, net			$ 41,232	$ 124,095	$ 27,010
Total revenues			1,316,232	1,027,336	551,234
Expenses
Employee compensation and benefits			437,269	349,192	157,855
Depreciation and amortization			81,448	55,570	34,938
Professional fees			25,596	46,662	14,072
Business development			9,763	4,609	23
Other			39,814	43,094	23,073
Total expenses			1,231,045	1,002,358	524,807
Income from continuing operations before income taxes			85,187	24,978	26,427
Income tax benefit			22,753	(101,114)	10,276
Income from continuing operations, net of tax			62,434	126,092	16,151
Net income	209,393	(89,305)	61,102	126,172	16,151
Parent Company [Member]
Revenues
Investment income and other, net			3,415	3,717	5,981
Total revenues			3,415	3,717	5,981
Expenses
Employee compensation and benefits			50,256	31,970	(12,251)
Interest			11,939	28,476	1,195
Depreciation and amortization			0	698	1,395
Professional fees			9,211	30,488	4,997
Business development			3,625	0	0
Other			25,460	29,829	1,493
Total expenses			100,491	121,461	(3,171)
Income from continuing operations before income taxes			(97,076)	(117,744)	9,152
Income tax benefit			(35,972)	(120,761)	0
Income from continuing operations, net of tax			(61,104)	3,017	9,152
Equity in earnings of subsidiaries			122,206	123,155	6,999
Net income			$ 61,102	$ 126,172	$ 16,151