Loans To And Investments In Associated Companies (Schedule Of Loans to and Investments In Associated Companies) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			$ 2,125,098	$ 1,757,369
Income (losses) related to associated companies	$ 14,104	$ 51,890	(114,470)	71,942
Income (losses) related to associated companies classified as other revenues	28,232	(7,836)	54,495	(31,252)
Contributions to (distributions from) associated companies, net			151,416	(64,632)
Other, including foreign exchange and unrealized gains (losses)			22,884	3,910
Loans to and investments in associated companies, ending balance	2,239,423	1,737,337	2,239,423	1,737,337
Noncontrolling interests	188,858		188,858		$ 175,549
Jefferies Finance [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			490,464	528,575
Income (losses) related to associated companies			0	0
Income (losses) related to associated companies classified as other revenues	25,211	(15,675)	50,176	(38,481)
Contributions to (distributions from) associated companies, net			0	(19,300)
Other, including foreign exchange and unrealized gains (losses)			0	0
Loans to and investments in associated companies, ending balance	540,640	470,794	540,640	470,794
Jefferies LoanCore [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			154,731	288,741
Income (losses) related to associated companies			0	0
Income (losses) related to associated companies classified as other revenues	4,042	8,201	6,374	8,014
Contributions to (distributions from) associated companies, net			56,950	(138,622)
Other, including foreign exchange and unrealized gains (losses)			0	0
Loans to and investments in associated companies, ending balance	218,055	158,133	218,055	158,133
Berkadia [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			184,443	190,986
Income (losses) related to associated companies	16,186	20,398	33,140	33,452
Income (losses) related to associated companies classified as other revenues			0	0
Contributions to (distributions from) associated companies, net			(4,567)	(40,748)
Other, including foreign exchange and unrealized gains (losses)			32	207
Loans to and investments in associated companies, ending balance	213,048	183,897	213,048	183,897
FXCM [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			336,258
Income (losses) related to associated companies	(12,115)	0	(162,015)	0
Income (losses) related to associated companies classified as other revenues			0
Contributions to (distributions from) associated companies, net			0
Other, including foreign exchange and unrealized gains (losses)			87
Loans to and investments in associated companies, ending balance	174,330		174,330
Garcadia [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			185,815	172,660
Income (losses) related to associated companies	12,677	13,941	25,971	29,268
Income (losses) related to associated companies classified as other revenues			0	0
Contributions to (distributions from) associated companies, net			(29,407)	(14,777)
Other, including foreign exchange and unrealized gains (losses)			0	0
Loans to and investments in associated companies, ending balance	182,379	187,151	182,379	187,151
Linkem [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			154,000	150,149
Income (losses) related to associated companies	(8,876)	(6,673)	(17,024)	(14,873)
Income (losses) related to associated companies classified as other revenues			0	0
Contributions to (distributions from) associated companies, net			31,996	33,297
Other, including foreign exchange and unrealized gains (losses)			22,765	3,462
Loans to and investments in associated companies, ending balance	191,737	172,035	191,737	172,035
HomeFed Corporation [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			302,231	275,378
Income (losses) related to associated companies	9,348	23,634	9,684	22,346
Income (losses) related to associated companies classified as other revenues			0	0
Contributions to (distributions from) associated companies, net			31,316	0
Other, including foreign exchange and unrealized gains (losses)			0	0
Loans to and investments in associated companies, ending balance	343,231	297,724	343,231	297,724
Golden Queen Mining Company, LLC [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			111,302	114,323
Income (losses) related to associated companies	(412)	(1,309)	(1,709)	(1,664)
Income (losses) related to associated companies classified as other revenues			0	0
Contributions to (distributions from) associated companies, net			(53)	0
Other, including foreign exchange and unrealized gains (losses)			0	0
Loans to and investments in associated companies, ending balance	109,540	112,659	109,540	112,659
Noncontrolling interests	32,300		32,300		32,800
54 Madison [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			161,400	0
Income (losses) related to associated companies	(3,556)	1,256	(4,164)	2,483
Income (losses) related to associated companies classified as other revenues			0	0
Contributions to (distributions from) associated companies, net			26,281	115,499
Other, including foreign exchange and unrealized gains (losses)			0	3,642
Loans to and investments in associated companies, ending balance	183,517	121,624	183,517	121,624
Noncontrolling interests	107,700		107,700		$ 95,300
Other [Member]
Loans to and Investments in Associated Companies Rollforward [Roll Forward]
Loans to and investments in associated companies, beginning balance			44,454	36,557
Income (losses) related to associated companies	852	643	1,647	930
Income (losses) related to associated companies classified as other revenues	(1,021)	(362)	(2,055)	(785)
Contributions to (distributions from) associated companies, net			38,900	19
Other, including foreign exchange and unrealized gains (losses)			0	(3,401)
Loans to and investments in associated companies, ending balance	$ 82,946	$ 33,320	$ 82,946	$ 33,320