Merger of GETCO and Knight - Assets Acquired and Liabilties Assumed (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Jul. 02, 2013 Knight [Member]
Identifiable Net Assets
Cash and cash equivalents			$ 509,133
Cash and securities segregated under federal and other regulations			203,045
Financial instruments owned			1,937,929
Securities borrowed			1,158,981
Receivable from brokers, dealers and clearing organizations			1,366,974
Fixed assets and leasehold improvements, net			84,596
Investments			106,353
Intangible assets			156,800
Assets within discontinued operations			5,607,063
Other assets			211,735
Total Assets			11,342,609
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]
Financial instruments sold, not yet purchased			1,512,983
Collateralized financings			1,166,211
Payable to brokers, dealers and clearing organizations			635,914
Payable to customers			527,918
Accrued compensation expense			107,409
Accrued expenses and other liabilities			139,624
Liabilities within discontinued operations			5,518,168
Long-term debt			375,000
Total Liabilities			9,983,227
Total identified assets acquired, net of assumed liabilities			1,359,382
Goodwill	18,398	4,645	13,753
Total Purchase Price			$ 1,373,135