Segment Information (Schedule Of Segment Reporting Information, By Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Net Revenues	$ 2,867,982	$ 2,015,106
Total consolidated pre-tax income from continuing operations	398,097	(301,963)
Depreciation and amortization	49,510	49,610
Unrealized and realized gains (losses), interest income and fees	416,501	(102,483)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Revenues	2,366,532	2,002,093
Total consolidated pre-tax income from continuing operations	174,869	(176,652)
Depreciation and amortization	38,867	38,159
All other [Member]
Segment Reporting Information [Line Items]
Net Revenues	501,450	13,013
Total consolidated pre-tax income from continuing operations	237,958	(110,597)
Depreciation and amortization	10,643	11,451
Parent Company interest [Member]
Segment Reporting Information [Line Items]
Total consolidated pre-tax income from continuing operations	(14,730)	(14,714)
Jefferies [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Revenues	797,386	300,786
Total consolidated pre-tax income from continuing operations	132,270	(245,797)
Depreciation and amortization	15,601	14,590
National Beef [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Revenues	1,561,456	1,634,451
Total consolidated pre-tax income from continuing operations	57,103	21,409
Depreciation and amortization	22,399	22,626
Corporate and Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Revenues	7,690	66,856
Total consolidated pre-tax income from continuing operations	(14,504)	47,736
Depreciation and amortization	867	943
FXCM [Member]
Segment Reporting Information [Line Items]
Unrealized and realized gains (losses), interest income and fees	$ 10,900	$ (53,200)