Assets of Businesses Held for Sale & Sales of Businesses - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2018	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Professional fees			$ 4,139	$ 4,406	$ 15,497	$ 21,281
Intangible assets, net of accumulated amortization			8,200		8,200			$ 26,000
Gain from sale of assets to third party					36,195	$ 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		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
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 | Retail Options Market Making Business
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
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
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			$ 59,900		59,900
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					$ 63,200