Warrants and Stock Repurchases - Additional Information (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Mar. 10, 2015	Jul. 01, 2013
Class of Stock [Line Items]
Warrants received (in shares)	4,472		6,182
Warrants repurchased (in shares)	1,077	1,500
Warrants repurchased	$ 2,900	$ 1,000
KCG Class A Common Stock repurchased	28,849
Remaining authorized repurchase amount	$ 136,800
Senior Secured Notes | 6.875% Senior Secured Notes
Class of Stock [Line Items]
Interest rate (percent)	6.875%	6.875%	6.875%	6.875%
GETCO
Class of Stock [Line Items]
Warrants received (in shares)					24,300
Common Class A
Class of Stock [Line Items]
KCG Class A Common Stock repurchased (in shares)	900	1,800
KCG Class A Common Stock repurchased	$ 13,300	$ 20,500