Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Cash Flows (Parent Only) (Details) - USD ($) $ in Thousands			6 Months Ended		12 Months Ended
	Mar. 16, 2015	Mar. 10, 2015	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 05, 2013
Cash flows from operating activities
Net income			$ 209,393	$ (89,305)	$ 249,104	$ 61,102	$ 126,172
Adjustments to reconcile net income to net cash provided by operating activities
Stock-based compensation					84,663	58,940	64,286
Debt discount accretion and other debt related expenses					12,829	13,217	17,332
Amortization expense					35,244	35,592	19,211
Decrease (increase)in operating assets
Subordinated loan receivable					265,763	14,468	(129,035)
Deferred tax asset					9,976	19,397	(103,499)
Other assets					(26,340)	(6,459)	25,785
(Decrease) increase in operating liabilities
Accrued compensation expense					21,726	(29,536)	24,441
Accrued expenses and other liabilities					(37,545)	(40,583)	(64,470)
Net cash provided by operating activities					12,984	242,524	141,204
Cash flows from investing activities
Purchases of fixed assets and leasehold improvements					(34,581)	(34,139)	(25,147)
Net cash (used in) provided by investing activities					328,478	11,472	582,892
Cash flows from financing activities
Proceeds from issuance of Credit Agreement					0	0	535,000
Partial repayment of Credit Agreement					0	(235,000)	(300,000)
Proceeds from issuance of 6.875% Senior Secured Notes, net					494,810	0	0
Proceeds from issuance of 8.25% Senior Secured Notes					0	0	305,000
Repayment of 8.25% Senior Secured Notes					(305,000)	0	0
Repayment of Notes					0	0	(15,000)
Payment of debt issuance costs					(12,645)	0	(34,592)
Cost of common stock repurchased - Tender Offer					(330,000)	0	0
Cost of common stock repurchased					(63,194)	(111,585)	(11,324)
Issuance of equity to General Atlantic					0	0	55,000
Payment to former Knight Capital Group, Inc. stockholders					0	0	(720,000)
Repayment of Knight Convertible Notes	$ (119,300)				(117,259)	0	0
Funding of collateral account for Knight Convertible Notes					0	0	(117,259)
Release of collateral account for Knight Convertible Notes					0	0	117,259
Members distributions					0	0	(21,002)
Stock options exercised					1,247	0	0
Warrants exercised					532	0	0
Cost of warrants repurchased					(4,441)	0	0
Income tax (provision) benefit related to stock-based compensation					2,647	0	0
Net cash used in financing activities					(337,336)	(349,925)	(478,811)
Increase (decrease) in cash and cash equivalents					2,545	(95,513)	246,650
Cash and cash equivalents at beginning of period				427,631	578,768	674,281	427,631
Cash and cash equivalents at end of period			674,281		581,313	578,768	674,281
Supplemental disclosure of cash flow information:
Cash paid for interest					81,349	76,003	56,227
Cash paid for income taxes					$ 124,461	16,975	10,198
Senior Secured Notes | 6.875% Senior Secured Notes
Cash flows from financing activities
Proceeds from issuance of 8.25% Senior Secured Notes		$ 494,800
Supplemental disclosure of cash flow information:
Interest rate		6.875%			6.875%
Senior Secured Notes | 8.25% Senior Secured Notes
Supplemental disclosure of cash flow information:
Interest rate					8.25%			8.25%
Parent Company
Cash flows from operating activities
Net income					$ 249,104	61,102	126,172
Adjustments to reconcile net income to net cash provided by operating activities
Equity in earnings of subsidiaries, net of tax					(302,602)	(122,206)	(123,155)
Stock-based compensation					14,942	16,997	0
Debt discount accretion and other debt related expenses					12,103	12,548	16,931
Amortization expense					0	0	698
Dividends received from subsidiaries					85,323	224,524	396,425
Decrease (increase)in operating assets
Subordinated loan receivable					0	(30,000)	(250,000)
Deferred tax asset					60,262	6,019	(144,109)
Other assets					(17,255)	(25,897)	5,568
(Decrease) increase in operating liabilities
Accrued compensation expense					5,050	13,208	0
Accrued expenses and other liabilities					20,063	3,178	64,571
Net cash provided by operating activities					126,990	159,473	93,101
Cash flows from investing activities
Cash acquired upon acquisition of Knight Capital Group, Inc.					0	0	509,133
Cash received from sale of Urban Financial of America, LLC					0	0	85,406
Purchases of fixed assets and leasehold improvements					(2,972)	0	0
Capital contributions to subsidiaries					0	0	(164,975)
Net cash (used in) provided by investing activities					(2,972)	0	429,564
Cash flows from financing activities
Proceeds from issuance of Credit Agreement					0	0	535,000
Partial repayment of Credit Agreement					0	(235,000)	(300,000)
Proceeds from issuance of 6.875% Senior Secured Notes, net					494,810	0	0
Proceeds from issuance of 8.25% Senior Secured Notes					0	0	305,000
Repayment of 8.25% Senior Secured Notes					(305,000)	0	0
Repayment of Notes					0	0	(15,000)
Payment of debt issuance costs					(12,645)	0	(34,592)
Cost of common stock repurchased - Tender Offer					(330,000)	0	0
Cost of common stock repurchased					(63,194)	(111,585)	(11,324)
Issuance of equity to General Atlantic					0	0	55,000
Payment to former Knight Capital Group, Inc. stockholders					0	0	(720,000)
Repayment of Knight Convertible Notes					0	0	(257,741)
Funding of collateral account for Knight Convertible Notes					0	0	(117,259)
Release of collateral account for Knight Convertible Notes					0	0	117,259
Members distributions					0	0	(21,002)
Cash funding transactions with subsidiaries					123,308	236,795	80,500
Stock options exercised					1,247	0	0
Warrants exercised					532	0	0
Cost of warrants repurchased					(4,441)	0	0
Income tax (provision) benefit related to stock-based compensation					2,647	0	0
Net cash used in financing activities					(92,736)	(109,790)	(384,159)
Increase (decrease) in cash and cash equivalents					31,282	49,683	138,506
Cash and cash equivalents at beginning of period				$ 114,511	302,700	253,017	114,511
Cash and cash equivalents at end of period			$ 253,017		333,982	302,700	253,017
Supplemental disclosure of cash flow information:
Cash paid for interest					33,878	28,426	26,239
Cash paid for income taxes					124,461	15,456	365
Non-cash net funding financing activities with subsidiaries					$ 54,510	$ 131,840	$ 1,082,031