CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net income	$ 255,697	$ 249,104	$ 61,102
Loss from discontinued operations, net of tax	0	0	(1,332)
Income from continuing operations, net of tax	255,697	249,104	62,434
Adjustments to reconcile Income from continuing operations, net of tax to net cash provided by operating activities
Realized gain on sale of KCG Hotspot	0	(385,026)	0
Realized gain from the sale of substantially all of the investment in Bats	(364,404)	0	0
Depreciation and amortization	88,790	90,231	81,448
Stock and unit-based compensation	20,459	84,663	58,940
Realized gain on sale of assets and investments	(2,798)	(19,751)	0
Unrealized gain on investments	(11,715)	(10,173)	(36,456)
Deferred taxes	41,363	9,976	19,397
Writedown of assets and other real estate related charges	0	56,642	8,625
Other	11,984	12,305	12,958
(Increase) decrease in operating assets
Cash and cash equivalents segregated under federal and other regulations	0	361	(54,955)
Financial instruments owned, at fair value	(95,461)	265,763	14,468
Securities borrowed	(50,443)	(4,223)	(274,675)
Receivable from brokers, dealers and clearing organizations	(142,608)	507,623	(321,087)
Other assets	39,312	(26,340)	(6,459)
Increase (decrease) in operating liabilities
Financial instruments sold, not yet purchased, at fair value	(67,265)	(172,302)	120,207
Securities loaned	(90,745)	(244,368)	(25,486)
Financial instruments sold under agreements to repurchase	72,873	21,327	292,625
Other collateralized financing	100,000	0	0
Payable to brokers, dealers and clearing organizations	239,292	(402,285)	268,563
Payable to customers	5,816	(4,724)	92,096
Accrued compensation expense	(7,317)	21,726	(29,536)
Accrued expenses and other liabilities	(12,301)	(37,545)	(40,583)
Income taxes payable	89,023	0	0
Net cash provided by operating activities	119,552	12,984	242,524
Cash flows from investing activities
Cash received from sale of substantially all of the investment in Bats	170,258	0	0
Cash received from sale of KCG Hotspot, net of cash provided	6,552	360,928	0
Cash received from sale of assets	21,220	0	554
Cash received from sale of investments and redemptions from investments	6,691	34,620	58,660
Purchases of fixed assets and leasehold improvements	(102,130)	(34,581)	(34,139)
Capitalization of software development costs	(31,998)	(24,530)	(14,859)
Purchases of investments	(5,877)	(7,959)	(744)
Purchase of business, net of cash acquired	(2,251)	0	0
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
Repayment of 8.25% Senior Secured Notes	0	(305,000)	0
Repayment of convertible notes	0	(117,259)	0
Payment of debt issuance costs	0	(12,645)	0
Borrowings under capital lease obligations	7,497	0	5,892
Principal payments on capital lease obligations	(2,056)	(4,033)	(9,232)
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
Proceeds from issuance of 6.875% Senior Secured Notes	0	494,810	0
Partial repayment of Credit Agreement	0	0	(235,000)
Net cash used in financing activities	(130,231)	(337,336)	(349,925)
Effect of exchange rate changes on cash and cash equivalents	(865)	(1,581)	416
(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 - Contribution of fixed assets to joint venture	353	3,370	0
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
Non-cash financing activities - KCG Class A Common Stock and Warrants repurchased via GA Swap - See Footnote 9 Investments	275,113	0	0
Future Commission Merchant [Member]
Cash flows from investing activities
Cash received from sale of Futures Commission Merchant	$ 0	$ 0	$ 2,000