Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 169,047	$ 143,465
Accumulated amortization	(85,216)	(59,398)
Net carrying amount	83,831	84,067
Held for sale, intangibles	8,200	26,000
Technology
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	147,538	120,256
Accumulated amortization	(79,723)	(54,858)
Net carrying amount	67,815	65,398
Held for sale, intangibles	$ 8,200	$ 8,800
Amortization period	2 years	2 years
Trading rights
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 7,509	$ 9,209
Accumulated amortization	(474)	(679)
Net carrying amount	$ 7,035	8,530
Held for sale, intangibles		$ 17,200
Amortization period	4 years	5 years
Indefinite-lived intangible assets	$ 6,900	$ 6,900
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000	13,000
Accumulated amortization	(4,694)	(3,611)
Net carrying amount	$ 8,306	$ 9,389
Amortization period	6 years	7 years
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 1,000	$ 1,000
Accumulated amortization	(325)	(250)
Net carrying amount	$ 675	$ 750
Amortization period	7 years	7 years
Market Making
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 46,536	$ 46,681
Market Making | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	39,501	38,151
Market Making | Trading rights
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	7,035	8,530
Global Execution Services
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	29,627	31,585
Global Execution Services | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	20,646	21,446
Global Execution Services | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	8,306	9,389
Global Execution Services | Trade names
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	675	750
Corporate and Other | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 7,668	$ 5,801