CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net income	$ 61,102,000	$ 126,172,000	$ 16,151,000
(Loss) Income from discontinued operations, net of tax	(1,332,000)	80,000	0
Income from continuing operations, net of tax	62,434,000	126,092,000	16,151,000
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Non-cash gain from sale of Futures Commission Merchant	(116,000)	0	0
Non-cash gain on Knight Common Stock	0	(127,972,000)	0
Deferred tax benefit	19,397,000	(103,499,000)	0
Depreciation and amortization	81,448,000	55,570,000	34,938,000
Stock and unit-based compensation	58,940,000	64,286,000	12,320,000
Writedown and amortization of debt offering costs	13,217,000	17,332,000	276,000
Writedown of assets and lease loss accrual, net	8,625,000	14,748,000	0
Unrealized (gain) loss on investments	(36,456,000)	(4,539,000)	(25,754,000)
Deferred rent	930,000	58,000	0
Operating activities from discontinued operations	(1,073,000)	6,952,000	0
(Increase) decrease in operating assets
Cash and cash equivalents segregated under federal and other regulations	(54,955,000)	19,963,000	0
Financial instruments owned, at fair value	14,468,000	(129,035,000)	(414,537,000)
Securities borrowed	(274,675,000)	(146,145,000)	(30,679,000)
Receivable from brokers, dealers and clearing organizations	(321,087,000)	252,266,000	36,439,000
Other assets	(6,459,000)	25,785,000	1,332,000
(Decrease) increase in operating liabilities
Financial instruments sold, not yet purchased, at fair value	120,207,000	139,951,000	0
Securities loaned	(25,486,000)	106,339,000	0
Financial instruments sold under agreements to repurchase	292,625,000	95,950,000	372,023,000
Payable to brokers, dealers and clearing organizations	268,563,000	(185,992,000)	(34,752,000)
Payable to customers	92,096,000	(46,877,000)	0
Accrued compensation expense	(29,536,000)	24,441,000	(18,494,000)
Accrued expenses and other liabilities	(40,583,000)	(64,470,000)	(13,779,000)
Net cash provided by (used in) operating activities	242,524,000	141,204,000	(64,516,000)
Cash flows from investing activities
Proceeds and distributions from investments	58,660,000	3,251,000	72,196,000
Purchases of fixed assets and leasehold improvements	(34,139,000)	(25,147,000)	(31,389,000)
Capitalized software development costs	(14,859,000)	(2,556,000)	0
Purchases of investments	(744,000)	(158,000)	(87,308,000)
Investing activities from discontinued operations	0	12,963,000	0
Sale of trading rights	554,000	0	0
Net cash provided by (used in) investing activities	11,472,000	582,892,000	(46,501,000)
Cash flows from financing activities
Proceeds from issuance of Credit Agreement	0	535,000,000	0
Partial repayment of Credit Agreement	(235,000,000)	(300,000,000)	0
Proceeds from issuance of Senior Secured Notes	0	305,000,000	0
Repayment of notes	0	(15,000,000)	0
Payment of debt issuance cost	0	(34,592,000)	(1,375,000)
Issuance of equity to General Atlantic	0	55,000,000	0
Payment to former Knight Capital Group, Inc. stockholders	0	(720,000,000)	0
Repayment of Knight Convertible Notes	0	(257,741,000)	0
Funding of collateral account for Knight Convertible Notes	0	(117,259,000)	0
Payment out of collateral account for Knight Convertible Notes	0	117,259,000	0
Borrowings under capital lease obligations	5,892,000	0	15,558,000
Principal payments on capital lease obligations	(9,232,000)	(14,152,000)	(20,990,000)
Cost of common stock repurchased	(111,585,000)	(11,324,000)	0
Borrowings under secured credit facility	0	25,000,000	88,750,000
Repayment of secured credit facility	0	(25,000,000)	(88,750,000)
Members' distributions	0	(21,002,000)	(37,061,000)
Repurchase of members' interest	0	0	(25,173,000)
Net cash used in financing activities	(349,925,000)	(478,811,000)	(69,041,000)
Effect of exchange rate changes on cash and cash equivalents	416,000	1,365,000	0
(Decrease) increase in cash and cash equivalents	(95,513,000)	246,650,000	(180,058,000)
Cash and cash equivalents at beginning of period	674,281,000	427,631,000	607,689,000
Cash and cash equivalents at end of period	578,768,000	674,281,000	427,631,000
Supplemental disclosure of cash flow information:
Cash paid for interest	76,003,000	56,227,000	8,084,000
Cash paid for income taxes	16,975,000	10,198,000	21,789,000
Future Commission Merchant [Member]
Cash flows from investing activities
Cash received from sale of businesses	2,000,000	0	0
Knight Capital Group [Member]
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Non-cash gain on Knight Common Stock		(128,000,000)
Cash flows from investing activities
Cash acquired upon acquisition of Knight Capital Group, Inc.	0	509,133,000	0
Urban Financial America [Member]
Cash flows from investing activities
Cash received from sale of businesses	$ 0	$ 85,406,000	$ 0