Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands				3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 24, 2017 segment	Jul. 19, 2017 item	May 03, 2017 USD ($)	Sep. 30, 2017 segment	Jun. 30, 2017 USD ($) item segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Jul. 20, 2017 USD ($)	May 03, 2017 USD ($)	Dec. 31, 2016 USD ($)
Changes in goodwill
Number of businesses Company is managed and operated as | item		1			1
Number of reportable segments		1			1
Number of operating segments | segment	3			3
Balance at the beginning of the period					$ 715,379	$ 715,379
Additions						129,504
Balance at the end of the period						844,883	$ 715,379
Purchase price						5,594
Goodwill					715,379	715,379	715,379	$ 844,883			$ 715,379
Goodwill impairment						0	0
Gross Carrying Amount											111,900
Accumulated Amortization								123,408			110,908
Intangible assets								111,224			992
Teza Technologies strategic telecommunications assets
Changes in goodwill
Balance at the end of the period			$ 1,200
Purchase price			5,600
Goodwill			$ 1,200							$ 1,200
Intangibles										$ 2,000
KCG
Changes in goodwill
Balance at the end of the period						128,286
Goodwill						128,286		128,286	$ 128,286
Intangibles								174,995
Gross Carrying Amount								53,700
Accumulated Amortization								2,900
Intangible assets								50,800	174,995
Market Making
Changes in goodwill
Balance at the beginning of the period					657,985	657,985
Additions						97,307
Balance at the end of the period						755,292	657,985
Goodwill					657,985	657,985	657,985	755,292	$ 96,200		657,985
Execution Services
Changes in goodwill
Balance at the beginning of the period					57,394	57,394
Additions						32,197
Balance at the end of the period						89,591	57,394
Goodwill					$ 57,394	$ 57,394	$ 57,394	$ 89,591			$ 57,394