CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net income	$ 59,505	$ 252,076
Adjustments to reconcile Net income to net cash provided by operating activities
Realized gain on sale of KCG Hotspot	0	(385,026)
Depreciation and amortization	66,015	65,154
Stock and unit-based compensation	25,161	74,949
Realized gain on sale of assets	(36,195)	0
Realized gain on repurchase of debt and investments	(3,676)	0
Unrealized gain on investments	(10,116)	(9,366)
Deferred taxes	0	(25,781)
Other	7,512	51,964
(Increase) decrease in operating assets
Cash and cash equivalents segregated under federal and other regulations	0	(5,039)
Financial instruments owned, at fair value	(460,553)	(139,736)
Securities borrowed	(320,316)	12,550
Receivable from brokers, dealers and clearing organizations	(6,318)	178,109
Other assets	26,653	2,187
Increase (decrease) in operating liabilities
Financial instruments sold, not yet purchased, at fair value	109,383	34,317
Securities loaned	184,225	61,396
Financial instruments sold under agreements to repurchase	79,798	180,176
Other collateralized financings	50,000	0
Payable to brokers, dealers and clearing organizations	367,480	(361,012)
Payable to customers	11,851	(1,318)
Accrued compensation expense	(44,291)	5,585
Accrued expenses and other liabilities	(15,112)	42,329
Net cash provided by operating activities	91,006	33,514
Cash flows from investing activities
Cash received from sale of KCG Hotspot, net of cash provided	6,552	360,928
Cash received from sale of assets	21,220	0
Cash received from sale of investments and redemptions from investments	52,967	2,523
Purchases of fixed assets and leasehold improvements	(81,916)	(22,657)
Capitalization of software development costs	(25,212)	(15,786)
Purchases of investments	(3,330)	(2,418)
Purchase of business, net of cash acquired	(2,251)	0
Net cash (used in) provided by investing activities	(31,970)	322,590
Cash flows from financing activities
Repurchase of 6.875% Senior Secured Notes	(30,288)	0
Proceeds from issuance of 6.875% Senior Secured Notes, net	0	494,810
Repayment of 8.25% Senior Secured Notes	0	(305,000)
Repayment of convertible notes	0	(117,259)
Payment of debt issuance costs	0	(12,645)
Principal payments on capital lease obligations	(1,535)	(3,478)
Cost of common stock repurchased - Tender Offer	0	(330,000)
Cost of common stock repurchased	(87,167)	(33,250)
Stock options exercised	15	1,247
Warrants exercised	0	247
Cost of warrants repurchased	(15,909)	0
Income tax provision on stock awards exercised	0	257
Net cash used in financing activities	(134,884)	(305,071)
Effect of exchange rate changes on cash and cash equivalents	(3)	(1,651)
(Decrease) Increase in cash and cash equivalents	(75,851)	49,382
Cash and cash equivalents at beginning of period	581,313	578,768
Cash and cash equivalents at end of period	505,462	628,150
Supplemental disclosure of cash flow information:
Cash paid for interest	53,171	69,307
Cash paid for income taxes	15,960	78,164
Non-cash investing activities - Contribution of fixed assets to joint venture	$ 353	$ 4,364