Commitments, Contingencies And Guarantees (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended
	Aug. 31, 2014	Mar. 31, 2017	Dec. 31, 2016
Commitments Contingencies And Guarantees [Line Items]
Loan commitments outstanding to clients		$ 177,500,000
Fair value of mortgage-related and other purchase commitments		14,400,000
Fair value of derivative contracts meeting the definition of a guarantee		134,100,000
54 Madison [Member]
Commitments Contingencies And Guarantees [Line Items]
Maximum amount committed to invest as per agreement		225,000,000.0
Amount of contributed commitment		130,400,000
Approved unfunded commitment		36,000,000
Berkadia [Member]
Commitments Contingencies And Guarantees [Line Items]
Surety policy issued		$ 1,500,000,000
Reimbursement of losses incurred, maximum percentage		50.00%
Aggregate amount of commercial paper outstanding		$ 1,470,000,000
Jefferies Finance [Member]
Commitments Contingencies And Guarantees [Line Items]
Line of credit commitment to associated companies, funded portion		0	$ 0
Line of credit facility commitment of Jefferies		250,000,000	$ 250,000,000
Maximum amount payable under guarantee		17,800,000
Energy Partners I, LP [Member]
Commitments Contingencies And Guarantees [Line Items]
Maximum exposure to loss of guarantee		3,000,000
Jefferies [Member] | Standby Letters Of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Letters of credit		63,200,000
Other Subsidiaries [Member]
Commitments Contingencies And Guarantees [Line Items]
Letters of credit		13,600,000
Jefferies Capital Partners LLC And Its Private Equity Funds [Member]
Commitments Contingencies And Guarantees [Line Items]
Equity commitments		22,800,000
Other Investments [Member]
Commitments Contingencies And Guarantees [Line Items]
Equity commitments		19,500,000
Folger Hill Asset Management Llc [Member]
Commitments Contingencies And Guarantees [Line Items]
Revolving credit facility term	3 years
Entity line of credit facility	$ 20,000,000
Funded revolving credit facility		11,600,000
Empire Insurance Company [Member]
Commitments Contingencies And Guarantees [Line Items]
Contractual obligations		$ 18,000,000
Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Expiration period maximum		1 year