Segment Information - Schedule of Segment Reporting Information By Segment (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Segment Reporting Information [Line Items]
Net Revenues	$ 2,939,509		$ 2,896,522	$ 2,732,380	$ 2,867,982	$ 2,745,778		$ 2,676,375	$ 2,625,358	$ 2,015,106	$ 11,436,393		$ 10,062,617		$ 10,886,458
Income (loss) from continuing operations before income taxes											1,013,814		316,430		356,536
Depreciation and amortization expenses											208,910		211,593		224,133
Identifiable assets employed	47,169,108	[1]				45,071,307	[1]				47,169,108	[1]	45,071,307	[1]	46,331,184
Principal transactions											931,576		603,822		642,824
Deferred tax asset, net	743,811					1,461,815					743,811		1,461,815
Parent Company interest
Segment Reporting Information [Line Items]
Income (loss) from continuing operations before income taxes											(58,943)		(58,881)		(87,181)
Identifiable assets employed	19,891,264					19,626,186					19,891,264		19,626,186
Principal transactions											(9,754)		16,735		491,341
Deferred tax asset, net	205,773					346,684					205,773		346,684
FXCM
Segment Reporting Information [Line Items]
Income (loss) from continuing operations before income taxes											(154,500)		(54,600)		491,300
Principal transactions											23,200		(54,600)		491,300
Jefferies Plan
Segment Reporting Information [Line Items]
Deferred tax asset, net	213,000					337,600					213,000		337,600		320,200
Operating Segments
Segment Reporting Information [Line Items]
Net Revenues											10,628,892		9,536,888		9,956,674
Income (loss) from continuing operations before income taxes											856,190		390,916		(49,045)
Depreciation and amortization expenses											166,361		158,307		185,226
Identifiable assets employed	43,042,807					40,425,531					43,042,807		40,425,531		41,899,234
All other
Segment Reporting Information [Line Items]
Net Revenues											807,501		525,729		950,784
Income (loss) from continuing operations before income taxes											216,567		(15,605)		492,762
Depreciation and amortization expenses											42,549		53,286		38,907
Identifiable assets employed	4,196,622					4,728,457					4,196,622		4,728,457		4,581,673
Intercompany eliminations
Segment Reporting Information [Line Items]
Net Revenues											0		0		(21,000)
Identifiable assets employed	(70,321)					(82,681)					(70,321)		(82,681)		(149,723)
Jefferies Plan | Operating Segments
Segment Reporting Information [Line Items]
Net Revenues											3,207,097		2,421,055		2,476,133
Income (loss) from continuing operations before income taxes											527,804		43,508		119,165
Depreciation and amortization expenses											62,668		60,206		92,165
Identifiable assets employed	39,575,732					36,992,096					39,575,732		36,992,096		38,607,786
National Beef | Operating Segments
Segment Reporting Information [Line Items]
Net Revenues											7,358,948		7,027,243		7,402,419
Income (loss) from continuing operations before income taxes											407,312		329,022		(123,915)
Depreciation and amortization expenses											98,515		94,482		89,317
Identifiable assets employed	1,460,539					1,498,317					1,460,539		1,498,317		1,514,249
Corporate and other | Operating Segments
Segment Reporting Information [Line Items]
Net Revenues											62,847		88,590		78,122
Income (loss) from continuing operations before income taxes											(78,926)		18,386		(44,295)
Depreciation and amortization expenses											5,178		3,619		3,744
Identifiable assets employed	$ 2,006,536					$ 1,935,118					$ 2,006,536		$ 1,935,118		$ 1,777,199

[1] Total assets include assets related to variable interest entities of $382.9 million and $815.8 million at December 31, 2017 and 2016, respectively, and Total liabilities include liabilities related to variable interest entities of $1,031.0 million and $1,284.7 million at December 31, 2017 and 2016, respectively. See Note 9 for additional information related to variable interest entities.