Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Cash Flows (Parent Only) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 30, 2016	Sep. 30, 2015	Mar. 13, 2015	Mar. 10, 2015	Jul. 01, 2013	Jun. 05, 2013
Cash flows from operating activities
Net income	$ 255,697	$ 249,104	$ 61,102
Adjustments to reconcile net income to net cash provided by operating activities
Deferred taxes	41,363	9,976	19,397
Stock-based compensation	20,459	84,663	58,940
Amortization expense	34,356	35,244	35,592
Decrease (increase) in operating assets
Subordinated loan receivable	(95,461)	265,763	14,468
Other assets	39,312	(26,340)	(6,459)
Income taxes payable	89,023	0	0
(Decrease) increase in operating liabilities
Accrued compensation expense	(7,317)	21,726	(29,536)
Accrued expenses and other liabilities	(12,301)	(37,545)	(40,583)
Net cash provided by operating activities	119,552	12,984	242,524
Cash flows from investing activities
Purchases of investments	6,691	34,620	58,660
Purchases of fixed assets and leasehold improvements	(102,130)	(34,581)	(34,139)
Net cash provided by investing activities	62,465	328,478	11,472
Cash flows from financing activities
Repurchase of 6.875% Senior Secured Notes	(30,288)	0	0
Borrowings under capital lease obligations	7,497	0	5,892
Partial payment of Credit Agreement	0	0	(235,000)
Repayment of 8.25% Senior Secured Notes	0	(305,000)	0
Payment of debt issuance costs	0	(12,645)	0
Cost of common stock repurchased - Tender Offer	0	(330,000)	0
Cost of common stock repurchased	(91,240)	(63,194)	(111,585)
Stock options exercised	353	1,247	0
Warrants exercised	0	532	0
Cost of warrants repurchased	(15,909)	(4,441)	0
Income tax provision on stock awards exercised	1,412	2,647	0
Net cash used in financing activities	(130,231)	(337,336)	(349,925)
(Decrease) Increase in cash and cash equivalents	50,921	2,545	(95,513)
Cash and cash equivalents at beginning of period	581,313	578,768	674,281
Cash and cash equivalents at end of period	632,234	581,313	578,768
Supplemental disclosure of cash flow information:
Cash paid for interest	81,766	81,349	76,003
Cash paid for income taxes	16,200	124,461	16,975
Non-cash investing activities - Purchases of fixed assets and leasehold improvements that were paid for subsequent to year end	10,272	0	0
Non-cash investing activities - Compensation capitalized for internal use software that was paid subsequent to year end	$ 3,494	$ 0	0
Senior Secured Notes | 6.875% Senior Secured Notes
Supplemental disclosure of cash flow information:
Interest rate (percent)	6.875%	6.875%		6.875%		6.875%	6.875%
Senior Secured Notes | 8.25% Senior Secured Notes
Supplemental disclosure of cash flow information:
Interest rate (percent)	8.25%	8.25%			8.25%	8.25%		8.25%	8.25%
Parent Company
Cash flows from operating activities
Net income	$ 255,697	$ 249,104	61,102
Adjustments to reconcile net income to net cash provided by operating activities
Equity in earnings of subsidiaries, net of tax	(412,960)	(302,602)	(122,206)
Deferred taxes	17,322	0	0
Stock-based compensation	3,729	14,942	16,997
Debt discount accretion and other debt related expenses	3,327	12,103	12,548
Realized gain on repurchase of debt	(3,676)	0	0
Amortization expense	464	0	0
Dividends received from subsidiaries	95,000	85,323	224,524
Decrease (increase) in operating assets
Subordinated loan receivable	(20,000)	0	(30,000)
Deferred tax asset	(11,100)	60,262	6,019
Other assets	15,392	(17,255)	(25,897)
Income taxes payable	87,475	0	0
(Decrease) increase in operating liabilities
Accrued compensation expense	(1,930)	5,050	13,208
Accrued expenses and other liabilities	14,020	20,063	3,178
Net cash provided by operating activities	42,760	126,990	159,473
Cash flows from investing activities
Purchases of investments	(583)	0	0
Purchases of fixed assets and leasehold improvements	(75,579)	(2,972)	0
Capital contributions to subsidiaries	(11,466)	0	0
Net cash provided by investing activities	(87,628)	(2,972)	0
Cash flows from financing activities
Repurchase of 6.875% Senior Secured Notes	(30,288)	0	0
Borrowings under capital lease obligations	7,497	0	0
Partial payment of Credit Agreement	0	0	(235,000)
Proceeds from issuance of 6.875% Senior Secured Notes, net	0	494,810	0
Repayment of 8.25% Senior Secured Notes	0	(305,000)	0
Payment of debt issuance costs	0	(12,645)	0
Cost of common stock repurchased - Tender Offer	0	(330,000)	0
Cost of common stock repurchased	(91,240)	(63,194)	(111,585)
Cash funding transactions with subsidiaries	208,943	123,308	236,795
Stock options exercised	353	1,247	0
Warrants exercised	0	532	0
Cost of warrants repurchased	(15,909)	(4,441)	0
Income tax provision on stock awards exercised	1,412	2,647	0
Net cash used in financing activities	80,768	(92,736)	(109,790)
(Decrease) Increase in cash and cash equivalents	35,900	31,282	49,683
Cash and cash equivalents at beginning of period	333,982	302,700	253,017
Cash and cash equivalents at end of period	369,882	333,982	302,700
Supplemental disclosure of cash flow information:
Cash paid for interest	34,554	33,878	28,426
Cash paid for income taxes	16,200	124,461	15,456
Non-cash investing activities - Purchases of fixed assets and leasehold improvements that were paid for subsequent to year end	10,272	0	0
Non-cash investing activities - Compensation capitalized for internal use software that was paid subsequent to year end	632	0	0
Non-cash net funding financing activities with subsidiaries	$ 318,731	$ 54,510	$ 131,840