Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands		3 Months Ended		7 Months Ended		9 Months Ended
	May 03, 2017 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)	Jul. 19, 2017 item	Jul. 19, 2017 segment	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jul. 20, 2017 USD ($)	May 03, 2017 USD ($)
Changes in goodwill
Number of businesses Company is managed and operated as				1	1
Number of reportable segments | segment					1
Number of operating segments | segment		3
Balance at the beginning of the period						$ 715,379
Additions						144,219
Balance at the end of the period		$ 859,598				859,598
Purchase price						5,594
Goodwill		859,598				715,379
Goodwill impairment		0	$ 0			0	$ 0
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									$ 1,900
Decrease in goodwill		1,900
Increase in intangible assets		1,900
KCG
Changes in goodwill
Goodwill								$ 143,012
Intangibles								156,300
Market Making
Changes in goodwill
Balance at the beginning of the period						657,985
Additions						108,276
Balance at the end of the period		766,261				766,261
Goodwill		766,261				657,985		107,200
Execution Services
Changes in goodwill
Balance at the beginning of the period						57,394
Additions						35,943
Balance at the end of the period		93,337				93,337
Goodwill		$ 93,337				$ 57,394		$ 35,800