Segment Information (Schedule Of Segment Reporting Information, By Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net Revenues	$ 1,150,846	$ 857,223	$ 2,957,440	$ 3,020,610
Consolidated income before income taxes	272,692	25,548	347,970	412,799
Depreciation and amortization expenses	32,295	28,760	92,360	82,129
Parent Company interest
Segment Reporting Information [Line Items]
Consolidated income before income taxes	(14,755)	(14,737)	(44,251)	(44,201)
FXCM
Segment Reporting Information [Line Items]
Consolidated income before income taxes	(2,900)	(2,000)	(2,900)	(148,700)
Principal transactions	1,300	2,300	16,400	17,600
Reportable Segments
Segment Reporting Information [Line Items]
Net Revenues	783,441	804,824	2,434,163	2,386,224
Consolidated income before income taxes	71,591	111,801	281,214	326,278
Depreciation and amortization expenses	18,027	16,793	53,428	49,476
Reportable Segments | Jefferies Group
Segment Reporting Information [Line Items]
Net Revenues	774,749	802,909	2,419,410	2,381,967
Consolidated income before income taxes	86,154	128,112	336,922	383,094
Depreciation and amortization expenses	17,175	15,928	50,829	46,877
Reportable Segments | Corporate
Segment Reporting Information [Line Items]
Net Revenues	8,692	1,915	14,753	4,257
Consolidated income before income taxes	(14,563)	(16,311)	(55,708)	(56,816)
Depreciation and amortization expenses	852	865	2,599	2,599
All other
Segment Reporting Information [Line Items]
Net Revenues	367,405	52,399	523,277	634,386
Consolidated income before income taxes	215,856	(71,516)	111,007	130,722
Depreciation and amortization expenses	$ 14,268	$ 11,967	$ 38,932	$ 32,653