Discontinued Operations, Assets and Liabilities Held for Sale & Sales of Businesses - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Professional fees		$ 27,055	$ 25,596	$ 46,662
Impairment of intangible assets		15,000
Assets held for sale		25,999	$ 40,484
KCG Hotspot
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related to tax sharing arrangement	$ 70,000	$ 70,000
Sale period	3 years	3 years
Additional consideration, fair value		$ 64,200
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 | 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]
Additional consideration, fair value		$ 64,200
KCG Hotspot | Payment in 2018
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 | Annual Payments
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related to tax sharing arrangement	$ 6,600