Merger of GETCO and Knight - Proforma Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Business Acquisition [Line Items]
Revenues - Reported	$ 467,819	$ 130,602	$ 698,123	$ 425,241
Net income from continuing operations- Reported	119,854	14,612	35,838	34,924
Net income	226,837	9,807	137,532	24,556
Diluted earnings per share from continuing operations (in dollars per share)	$ 1.98	$ 0.21	$ 2.01	$ 0.49
Diluted earnings per share (in dollars per share)	$ 1.98	$ 0.21	$ 2.00	$ 0.49
Knight [Member]
Business Acquisition [Line Items]
Revenues	240,805
Income from continuing operations, before income taxes	21,164
Revenues - Reported	130,602		698,123	425,241
Revenues - Pro Forma	(120,210)		1,099,518	677,955
Net income from continuing operations- Reported	9,807		138,316	24,556
Net income from continuing operations - Pro Forma	(303,413)		(10,494)	(274,981)
Net income	9,807		137,532	24,556
Net income - Pro Forma	$ (387,350)		$ (39,554)	$ (350,831)
Diluted earnings per share from continuing operations (in dollars per share)	$ 0.21		$ 2.01	$ 0.49
Diluted earnings per share from continuing operations - Pro Forma	$ (6.54)		$ (0.15)	$ (5.54)
Diluted earnings per share (in dollars per share)	$ 0.21		$ 2.00	$ 0.49
Diluted earnings per share - Pro Forma	$ (8.35)		$ (0.57)	$ (7.07)