Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 20, 2017
Income taxes
Provision for (benefit from) income taxes	$ 94,266	$ 21,251	$ 18,439
Effective tax rate (as a percent)	83.30%	11.90%	8.60%
Current income taxes receivable	$ 115,200	$ 5,800
Alternative minimum tax credit carryforward	600
Deferred tax asset	125,760	193,859
Unrecognized tax benefits	7,300	0
VFH
Income taxes
Deferred tax asset	124,631	$ 192,961
KCG
Income taxes
Deferred tax asset	43,500			$ 23,900
KCG | Non-US
Income taxes
Net operating loss	$ 231,800