Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Operations and Comprehensive Income (Parent Only) (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Investment income and other, net			$ 421,384	$ 41,232	$ 124,095
Total revenues			1,599,110	1,316,232	1,027,336
Expenses
Employee compensation and benefits			405,609	437,269	349,192
Depreciation and amortization			90,231	81,448	55,570
Professional fees			27,055	25,596	46,662
Business development			8,479	9,763	4,609
Occupancy and equipment rentals			30,128	32,707	24,812
Other			38,375	39,814	43,094
Total expenses			1,219,148	1,231,045	1,002,358
Income tax benefit			130,858	22,753	(101,114)
Net income	$ 209,393	$ (89,305)	249,104	61,102	126,172
Comprehensive income			247,321	61,834	13,254
Parent Company
Revenues
Investment income and other, net			7,341	3,415	3,717
Total revenues			7,341	3,415	3,717
Expenses
Employee compensation and benefits			48,863	50,256	31,970
Debt interest expense			35,883	11,939	28,476
Depreciation and amortization			0	0	698
Professional fees			15,728	9,211	30,488
Business development			2,759	3,625	0
Occupancy and equipment rentals			2,059	0	0
Other			31,331	25,460	29,829
Total expenses			136,623	100,491	121,461
Loss before income taxes and equity in earnings of subsidiaries			(129,282)	(97,076)	(117,744)
Income tax benefit			(75,784)	(35,972)	(120,761)
(Loss) income before equity in earnings of subsidiaries			(53,498)	(61,104)	3,017
Equity in earnings of subsidiaries			302,602	122,206	123,155
Net income			249,104	61,102	126,172
Other comprehensive income (loss)			(1,783)	732	(112,918)
Comprehensive income			$ 247,321	$ 61,834	$ 13,254