Discontinued Operations, Assets of Businesses Held for Sale & Sales of Businesses - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Professional fees				$ 19,827	$ 27,055	$ 25,596
Intangible assets, net of accumulated amortization				8,200	26,000
Gain from sale of assets to third party				2,798	19,751	$ 0
Disposal Group, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Intangible assets, net of accumulated amortization				8,194	$ 25,999
KCG Hotspot
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale period		3 years			3 years
Annual payments for share in tax benefits	$ 6,600
KCG Hotspot | Disposal Group, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of business					$ 373,800
KCG Hotspot | Investment Income and Other, Net
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of business					385,000
KCG Hotspot | Investment Income and Other, Net | Disposal Group, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of business					385,000
KCG Hotspot | Investment Income and Other, Net | Retail Options Market Making Business | Disposal Group, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of assets to third party				2,900
KCG Hotspot | Professional Fees | Disposal Group, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Professional fees					6,700
KCG Hotspot | Employee Compensation and Benefits | Disposal Group, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Compensation costs					$ 4,500
KCG Hotspot | Other Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Receivables from sale of investment				60,500
KCG Hotspot | Scenario, Forecast
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related to tax sharing arrangement			$ 50,000
KCG Hotspot | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related to tax sharing arrangement		$ 70,000
Annual payments for share in tax benefits		$ 6,600
Remaining additional payment for share in tax benefits				$ 63,100