Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended			9 Months Ended		3 Months Ended	9 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Stock Appreciation Rights (SARs) [Member]	Sep. 30, 2013 Class B Units [Member]	Sep. 30, 2013 Class E Units [Member]	Sep. 30, 2013 Class E Units [Member]	Jul. 02, 2013 KCG Plan [Member]	Sep. 30, 2013 Restricted Stock [Member]	Sep. 30, 2013 Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grant								20,700,000
Percent of shares vesting over vesting period							3.33%
Vesting period					3 years		3 years		3 years	3 years
Unamortized compensation costs		$ 1,300,000		$ 7,200,000	$ 9,400,000
Award expiration period				2 years 2 months 12 days			5 years
Unamortized compensation related to restricted awards outstanding									79,600,000
Cost of unvested awards expected to be recognized over a weighted average life, years									2 years 6 months
Number of Stock Options, Exercised (in shares)	0
Employee compensation deferred tax asset		3,000,000	3,500,000
Accelerated amortization						$ 0	$ 3,500,000