Commitments, Contingencies And Guarantees (Narrative) (Details) plaintif in Thousands		1 Months Ended	12 Months Ended
	Dec. 30, 2004 USD ($)	Aug. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) plaintif	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Loss Contingencies [Line Items]
Rental expense (net of sublease rental income)			$ 84,000,000	$ 79,600,000	$ 63,900,000
Purchase commitments			752,400,000	$ 344,800,000
Loan commitments outstanding to clients			268,700,000
Fair value of mortgage-related and other purchase commitments			$ 238,600,000
Approximate number of plaintiffs | plaintif			124
Amount accrued for estimated probable losses in connection with litigation			$ 50,000,000
Fair value of derivative contracts meeting the definition of a guarantee			394,800,000
Lease obligations			30,700,000
Jefferies Capital Partners LLC And Its Private Equity Funds [Member]
Loss Contingencies [Line Items]
Equity commitments			23,600,000
Folger Hill Asset Management LLC [Member]
Loss Contingencies [Line Items]
Entity line of credit facility, Term		3 years
Revolving credit facility		$ 20,000,000
Amount funded under credit facility			7,400,000
Other Investments [Member]
Loss Contingencies [Line Items]
Equity commitments			4,400,000
National Beef [Member]
Loss Contingencies [Line Items]
Service charge commitments for city improvements	$ 19,300,000
Service charge commitment for city improvements, duration of payments	20 years
Service charge commitments for city improvements, remaining amount			6,600,000
Service charge commitment future payment for city improvements, 2016			800,000
Service charge commitment future payment for city improvements, 2017			800,000
Service charge commitment future payment for city improvements, 2018			800,000
Service charge commitment for city improvements remaining balance to be paid in subsequent years			4,100,000
Purchase commitments			68,500,000
Letters of credit			19,800,000
54 Madison [Member]
Loss Contingencies [Line Items]
Equity commitments			225,000,000
Funded equity commitments			38,400,000
Jefferies Finance [Member]
Loss Contingencies [Line Items]
Funded equity commitments			497,400,000
Maximum amount payable under guarantee			$ 21,800,000.0
Berkadia [Member]
Loss Contingencies [Line Items]
Reimbursement of losses incurred, maximum percentage			50.00%
Surety policy issued			$ 2,500,000,000
Aggregate amount of commercial paper outstanding			2,470,000,000
Jefferies [Member] | Standby Letters Of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit			$ 33,100,000
Expiration period maximum			1 year
Other Subisidiaries [Member]
Loss Contingencies [Line Items]
Letters of credit			$ 23,100,000
Minimum [Member]
Loss Contingencies [Line Items]
Noncancellable operating lease terms			1 year
Maximum [Member]
Loss Contingencies [Line Items]
Noncancellable operating lease terms			30 years