Commitments, Contingencies and Guarantees (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended
	Aug. 31, 2014	Mar. 31, 2018	Apr. 01, 2018	Dec. 31, 2017
Commitments Contingencies And Guarantees [Line Items]
Loan commitments outstanding to clients		$ 66,800,000
Fair value of mortgage-related and other purchase commitments		1,100,000
Fair value of derivative contracts meeting the definition of a guarantee		391,400,000
Contractual obligations		$ 6,956,500,000
Standby Letters of Credit
Commitments Contingencies And Guarantees [Line Items]
Expiration period maximum		1 year
Berkadia
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
Commitments Contingencies And Guarantees [Line Items]
Line of credit facility, commitment of Jefferies, funded		121,200,000		$ 0
Line of credit facility commitment of Jefferies		$ 250,000,000.0		$ 250,000,000.0
Folger Hill
Commitments Contingencies And Guarantees [Line Items]
Revolving credit facility term	3 years
Entity line of credit facility	$ 20,000,000
Number of days prior written notice for optional termination right		10 days
Funded revolving credit facility		$ 10,200,000
Jefferies | Standby Letters of Credit
Commitments Contingencies And Guarantees [Line Items]
Letters of credit		52,000,000
Other subsidiaries
Commitments Contingencies And Guarantees [Line Items]
Letters of credit		15,000,000
Jefferies Capital Partners LLC and Its Private Equity Funds
Commitments Contingencies And Guarantees [Line Items]
Equity commitments		21,800,000
Other Investments
Commitments Contingencies And Guarantees [Line Items]
Equity commitments		42,400,000
Empire Insurance Company
Commitments Contingencies And Guarantees [Line Items]
Contractual obligations		$ 6,400,000
Subsequent Event | Maximum | Other Investments
Commitments Contingencies And Guarantees [Line Items]
Equity commitments			$ 500,000,000.0