Commitments, Contingencies and Guarantees (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Commitments Contingencies And Guarantees [Line Items]
Loan commitments outstanding to clients	$ 86,300,000
Fair value of derivative contracts meeting the definition of a guarantee	216,900,000
Contractual obligations	$ 6,599,400,000
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	0	$ 0
Line of credit facility commitment of Jefferies	250,000,000.0	$ 250,000,000.0
Jefferies Group | Standby Letters of Credit
Commitments Contingencies And Guarantees [Line Items]
Letters of credit	51,800,000
Other subsidiaries
Commitments Contingencies And Guarantees [Line Items]
Letters of credit	1,100,000
Jefferies Capital Partners LLC and Its Private Equity Funds
Commitments Contingencies And Guarantees [Line Items]
Equity commitments	18,100,000
Other Investments
Commitments Contingencies And Guarantees [Line Items]
Equity commitments	316,700,000
Alternative Asset Management Platforms
Commitments Contingencies And Guarantees [Line Items]
Equity commitments	$ 280,000,000