Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Operations and Comprehensive Income (Parent Only) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Investment income and other, net	$ 393,365	$ 421,384	$ 41,232
Total revenues	1,454,412	1,599,110	1,316,232
Expenses
Employee compensation and benefits	295,120	405,609	437,269
Depreciation and amortization	88,790	90,231	81,448
Professional fees	19,827	27,055	25,596
Business development	5,324	8,479	9,763
Occupancy and equipment rentals	37,875	30,128	32,707
Communications and data processing	147,986	139,263	150,595
Other	35,346	34,839	36,149
Total expenses	1,057,984	1,219,148	1,231,045
Income tax expense (benefit)	140,731	130,858	22,753
Net income	255,697	249,104	61,102
Comprehensive income	257,594	247,321	61,834
Parent Company
Revenues
Investment income and other, net	9,469	7,341	3,415
Total revenues	9,469	7,341	3,415
Expenses
Employee compensation and benefits	32,660	48,863	50,256
Debt interest expense	19,823	39,419	15,604
Depreciation and amortization	633	0	0
Professional fees	10,188	15,728	9,211
Business development	964	2,759	3,625
Occupancy and equipment rentals	12,904	2,059	0
Communications and data processing	2,008	0	0
Other	17,898	27,795	21,795
Total expenses	97,078	136,623	100,491
Loss before income taxes and equity in earnings of subsidiaries	(87,609)	(129,282)	(97,076)
Income tax expense (benefit)	69,654	(75,784)	(35,972)
(Loss) income before equity in earnings of subsidiaries	(157,263)	(53,498)	(61,104)
Equity in earnings of subsidiaries	412,960	302,602	122,206
Net income	255,697	249,104	61,102
Other comprehensive income (loss)	1,897	(1,783)	732
Comprehensive income	$ 257,594	$ 247,321	$ 61,834