Commitments, Contingencies And Guarantees (Narrative) (Details) plaintif in Thousands, € in Millions		1 Months Ended		12 Months Ended
	Dec. 30, 2004 USD ($)	Jan. 31, 2017 EUR (€)	Aug. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) plaintif	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jan. 31, 2017 USD ($)	Jan. 31, 2017 EUR (€)	Mar. 18, 2015 USD ($)
Loss Contingencies [Line Items]
Rental expense (net of sublease rental income)				$ 80,400,000	$ 84,000,000	$ 79,600,000
Loan commitments outstanding to clients				$ 182,100,000
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				$ 313,100,000
Jefferies Capital Partners LLC And Its Private Equity Funds [Member]
Loss Contingencies [Line Items]
Equity commitments				23,100,000
Account Managed by Folger Hill Asia [Member]
Loss Contingencies [Line Items]
Equity commitments				125,000,000
Other Various Investments [Member]
Loss Contingencies [Line Items]
Equity commitments				23,400,000
Folger Hill Asset Management LLC [Member]
Loss Contingencies [Line Items]
Entity line of credit facility, Term			3 years
Entity line of credit facility			$ 20,000,000
Amount funded under credit facility				9,400,000
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				5,800,000
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				103,800,000
Letters of credit				12,500,000
54 Madison [Member]
Loss Contingencies [Line Items]
Maximum amount committed to invest as per agreement				225,000,000.0
Amount of contributed commitment				114,900,000
Approved unfunded commitment				37,400,000
Jefferies Finance [Member]
Loss Contingencies [Line Items]
Line of credit commitment to associated companies, funded portion				0	19,300,000
Line of credit facility commitment of Jefferies				250,000,000	$ 250,000,000
Maximum amount payable under guarantee				18,100,000
Energy Partners I, LP [Member]
Loss Contingencies [Line Items]
Maximum exposure to loss of guarantee				3,000,000
Jefferies [Member] | Standby Letters Of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit				$ 33,300,000
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				1,470,000,000
Other Subisidiaries [Member]
Loss Contingencies [Line Items]
Letters of credit				$ 13,600,000
Minimum [Member]
Loss Contingencies [Line Items]
Noncancellable operating lease terms				1 year
Maximum [Member]
Loss Contingencies [Line Items]
Noncancellable operating lease terms				23 years
Subsequent Event [Member]
Loss Contingencies [Line Items]
Payments to Acquire Equity Method Investments							$ 75,000,000
Subsequent Event [Member] | Preferred Equity Financing for Linkem [Member]
Loss Contingencies [Line Items]
Equity investments in related funds | €								€ 30
Subsequent Event [Member] | Linkem [Member]
Loss Contingencies [Line Items]
Preferred Equity Financing | €		€ 100
Empire Insurance Company [Member]
Loss Contingencies [Line Items]
Lease obligations				$ 20,800,000
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Debt instrument, term				1 year