Loans To And Investments In Associated Companies (Schedule of Loans to and Investments in Associated Companies) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 01, 2016	Dec. 31, 2009
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 1,757,369	$ 1,712,568	$ 1,258,341
Income related to associated companies	154,598	110,281	138,527
Income (losses) related to associated companies classified as other revenues	17,184	75,717	90,242
Contributions to (distributions from) associated companies, net	(132,070)	(120,860)	26,878
Other, including foreign exchange and unrealized gain (losses)	328,017	(20,337)	198,580
Loans to and investments in associated companies ending balance	2,125,098	1,757,369	1,712,568
Noncontrolling interests	175,549	64,679
Jefferies Finance [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	528,575	508,891	470,537
Income related to associated companies	0	0	0
Income (losses) related to associated companies classified as other revenues	(1,761)	40,884	72,701
Contributions to (distributions from) associated companies, net	(36,350)	(21,200)	(34,347)
Other, including foreign exchange and unrealized gain (losses)	0	0	0
Loans to and investments in associated companies ending balance	$ 490,464	528,575	508,891
Percentage of ownership owned	50.00%
Jefferies LoanCore [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 288,741	258,947	224,037
Income related to associated companies	0	0	0
Income (losses) related to associated companies classified as other revenues	21,221	36,554	18,793
Contributions to (distributions from) associated companies, net	(155,231)	(6,760)	16,117
Other, including foreign exchange and unrealized gain (losses)	0	0	0
Loans to and investments in associated companies ending balance	$ 154,731	288,741	258,947
Percentage of ownership owned	48.50%
Berkadia [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 190,986	208,511	182,573
Income related to associated companies	94,201	78,092	101,187
Income (losses) related to associated companies classified as other revenues	0	0	0
Contributions to (distributions from) associated companies, net	(100,766)	(89,560)	(72,721)
Other, including foreign exchange and unrealized gain (losses)	22	(6,057)	(2,528)
Loans to and investments in associated companies ending balance	$ 184,443	190,986	208,511
Percentage of ownership owned	50.00%				50.00%
Garcadia [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 172,660	167,939	120,017
Income related to associated companies	52,266	53,182	49,416
Income (losses) related to associated companies classified as other revenues	0	0	0
Contributions to (distributions from) associated companies, net	(39,111)	(48,461)	(1,494)
Other, including foreign exchange and unrealized gain (losses)	0	0	0
Loans to and investments in associated companies ending balance	$ 185,815	172,660	167,939
Percentage of ownership owned	75.00%
Linkem [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 150,149	159,054	173,577
Income related to associated companies	(22,867)	(15,577)	(14,633)
Income (losses) related to associated companies classified as other revenues	0	0	0
Contributions to (distributions from) associated companies, net	33,303	21,138	18,390
Other, including foreign exchange and unrealized gain (losses)	(6,585)	(14,466)	(18,280)
Loans to and investments in associated companies ending balance	$ 154,000	150,149	159,054
Percentage of ownership owned	42.00%
HomeFed Corporation [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 275,378	271,782	52,923
Income related to associated companies	23,893	3,596	3,150
Income (losses) related to associated companies classified as other revenues	0	0	0
Contributions to (distributions from) associated companies, net	2,960	0	0
Other, including foreign exchange and unrealized gain (losses)	0	0	215,709
Loans to and investments in associated companies ending balance	$ 302,231	275,378	271,782
Percentage of ownership owned	65.00%
Golden Queen Mining Company, LLC [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	$ 114,323	103,598	0
Income related to associated companies	(3,021)	(1,775)	(1,402)
Income (losses) related to associated companies classified as other revenues	0	0	0
Contributions to (distributions from) associated companies, net	0	12,500	105,000
Other, including foreign exchange and unrealized gain (losses)	0	0	0
Loans to and investments in associated companies ending balance	$ 111,302	114,323	103,598
Percentage of ownership owned	35.00%
Noncontrolling interests	$ 32,800	33,700	33,700
Other [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	36,557	33,846	34,677
Income related to associated companies	3,952	(7,237)	809
Income (losses) related to associated companies classified as other revenues	(2,276)	(1,721)	(1,252)
Contributions to (distributions from) associated companies, net	9,622	11,483	(4,067)
Other, including foreign exchange and unrealized gain (losses)	(3,401)	186	3,679
Loans to and investments in associated companies ending balance	44,454	36,557	$ 33,846
FXCM [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	0
Income related to associated companies	1,919
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 gain (losses)	334,339
Loans to and investments in associated companies ending balance	336,258	0
54 Madison [Member]
Equity Method Investment [Roll Forward]
Loans to and investments in associated companies beginning balance	0
Income related to associated companies	4,255
Income (losses) related to associated companies classified as other revenues	0
Contributions to (distributions from) associated companies, net	153,503
Other, including foreign exchange and unrealized gain (losses)	3,642
Loans to and investments in associated companies ending balance	$ 161,400	$ 0
Percentage of ownership owned	48.10%
Noncontrolling interests	$ 95,300
Investment in FXCM [Member]
Equity Method Investment [Roll Forward]
Percentage of ownership owned	49.90%			49.90%