Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended				6 Months Ended					3 Months Ended		6 Months Ended					3 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012	Jun. 30, 2013 Foreign currency transactions [Member]	Jun. 30, 2012 Foreign currency transactions [Member]	Jun. 30, 2013 Foreign currency transactions [Member]	Jun. 30, 2012 Foreign currency transactions [Member]	Jun. 30, 2013 Software [Member]	Jun. 30, 2013 Minimum [Member]	Jun. 30, 2013 Maximum [Member]	Mar. 31, 2013 Out of Period Adjustments [Member]	Jun. 30, 2013 Out of Period Adjustments [Member] Reverse Mortgage Business [Member]
Property, Plant and Equipment [Line Items]
Inventory valuation adjustment																		$ 7,000,000
Maximum maturity of short-term investments					90 days
Equity ownership	20.00%				20.00%				20.00%						20.00%	50.00%
Partnership interest															3.00%	5.00%
Investment gain (loss) and other, net										16,000	1,200,000	33,000	1,200,000
Fixed assets, useful life															3 years	7 years
Amortization period, in years														3 years	2 years	17 years
Over accrual of expenses																		2,000,000
Immaterial error correction, amount																		5,000,000
Net trading revenue	$ 176,499,000	[1]	$ 96,222,000	[1]	$ 325,337,000	[1]	$ 249,356,000	[1]									$ 3,100,000

[1] See Explanatory Note - All financial information is of Knight Capital Group, Inc. As of July 1, 2013, Knight Capital Group, Inc., became a wholly-owned subsidiary of KCG