Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 205,076	$ 178,697
Accumulated amortization	(114,630)	(94,762)
Net carrying amount	90,446	83,935
Held for sale, intangibles		8,200
Technology
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	183,567	157,188
Accumulated amortization	(108,347)	(88,874)
Net carrying amount	$ 75,220	68,314
Held for sale, intangibles		$ 8,200
Amortization period	2 years	2 years
Trading rights
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 7,509	$ 7,509
Accumulated amortization	(491)	(482)
Net carrying amount	$ 7,018	$ 7,027
Amortization period	3 years	4 years
Indefinite-lived intangible assets	$ 6,900
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000	$ 13,000
Accumulated amortization	(5,417)	(5,056)
Net carrying amount	$ 7,583	$ 7,944
Amortization period	5 years	6 years
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 1,000	$ 1,000
Accumulated amortization	(375)	(350)
Net carrying amount	$ 625	$ 650
Amortization period	6 years	7 years
Market Making
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 45,787	$ 46,563
Market Making | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	38,759	39,536
Market Making | Trading rights
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	7,028	7,027
Global Execution Services
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	28,241	29,288
Global Execution Services | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	20,033	20,694
Global Execution Services | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	7,583	7,944
Global Execution Services | Trade names
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	625	650
Corporate and Other | Technology
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 16,418	$ 8,084