Income Taxes (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes [Line Items]
Unrecognized tax benefits	$ 237.8		$ 226.4
Decrease of income tax provision	43.9
Interest included in unrecognized tax benefits	63.0		57.4
Tax benefit from repatriation of foreign earnings		$ 31.9
Increase of income tax expense related to Tax Cuts and Jobs Act Of 2017	3.8
Jefferies Group
Income Taxes [Line Items]
Unrecognized tax benefits	$ 184.0		$ 177.8