Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 84,067	$ 135,283
Held for sale, intangibles	25,999	34,696
KCG Hotspot
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 34,700
Technology
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	$ 8,800
Amortization period	2 years	3 years
Technology | KCG Hotspot
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 13,100
Trading rights
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	$ 17,200
Amortization period	5 years	7 years
Indefinite-lived intangible assets	$ 6,900	$ 6,900
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Amortization period	7 years	8 years
Customer relationships | KCG Hotspot
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 19,000
Trade names
Finite-Lived Intangible Assets [Line Items]
Amortization period	7 years	8 years
Trade names | KCG Hotspot
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 2,600
Operating Segments
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 143,465	192,272
Accumulated amortization	(59,398)	(56,989)
Operating Segments | Technology
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	120,256	115,804
Accumulated amortization	(54,858)	(36,562)
Net carrying amount	65,398	79,242
Operating Segments | Trading rights
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	9,209	62,468
Accumulated amortization	(679)	(18,110)
Net carrying amount	8,530	44,358
Operating Segments | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000	13,000
Accumulated amortization	(3,611)	(2,167)
Net carrying amount	9,389	10,833
Operating Segments | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,000	1,000
Accumulated amortization	(250)	(150)
Net carrying amount	750	850
Operating Segments | Market Making
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	46,681	94,900
Operating Segments | Market Making | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	38,151	50,542
Operating Segments | Market Making | Trading rights
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	8,530	44,358
Operating Segments | Global Execution Services
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	31,585	29,883
Operating Segments | Global Execution Services | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	21,446	18,200
Operating Segments | Global Execution Services | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	9,389	10,833
Operating Segments | Global Execution Services | Trade names
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	750	850
Corporate, Non-Segment | Corporate and Other | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 5,801	$ 10,500