Assets of Business Held for Sale & Sales of Businesses - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Intangible assets, net of accumulated amortization							$ 8,200
Gain from sale of assets to third party				$ 4,834	$ 2,798
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
KCG Hotspot
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale period			3 years
Annual payments for share in tax benefits	$ 6,500	$ 6,600
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 | Other Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Receivables from sale of investment	$ 54,100			54,100
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				$ 56,800