Commitments, Contingencies and Guarantees (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2014	Sep. 30, 2017	Sep. 30, 2017	Oct. 31, 2017	Dec. 31, 2016
Commitments Contingencies And Guarantees [Line Items]
Loan commitments outstanding to clients		$ 64,900,000	$ 64,900,000
Fair value of mortgage-related and other purchase commitments		44,500,000	44,500,000
Contractual obligations for the remainder of fiscal 2017		6,086,000,000	6,086,000,000
Contractual obligation for fiscal 2018		438,800,000	438,800,000
Fair value of derivative contracts meeting the definition of a guarantee		245,500,000	245,500,000
Contractual obligations		7,350,000,000	7,350,000,000
54 Madison [Member]
Commitments Contingencies And Guarantees [Line Items]
Maximum amount committed to invest as per agreement		202,500,000.0	202,500,000.0
Amount of contributed commitment		134,400,000	134,400,000
Approved unfunded commitment		36,300,000	36,300,000
Berkadia [Member]
Commitments Contingencies And Guarantees [Line Items]
Surety policy issued		$ 1,500,000,000	$ 1,500,000,000
Reimbursement of losses incurred, maximum percentage		50.00%	50.00%
Aggregate amount of commercial paper outstanding		$ 1,470,000,000	$ 1,470,000,000
Jefferies Finance [Member]
Commitments Contingencies And Guarantees [Line Items]
Line of credit facility, commitment of Jefferies, funded		35,100,000	35,100,000		$ 0
Line of credit facility commitment of Jefferies		250,000,000.0	250,000,000.0		$ 250,000,000.0
Maximum amount payable under guarantee		200,000	200,000
Jefferies [Member] | Standby Letters Of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Letters of credit		54,000,000	54,000,000
Other Subsidiaries [Member]
Commitments Contingencies And Guarantees [Line Items]
Letters of credit		$ 15,000,000	15,000,000
Folger Hill [Member]
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	10,200,000
Jefferies Capital Partners LLC And Its Private Equity Funds [Member]
Commitments Contingencies And Guarantees [Line Items]
Equity commitments		22,000,000	22,000,000
Other Investments [Member]
Commitments Contingencies And Guarantees [Line Items]
Equity commitments		15,400,000	15,400,000
Empire Insurance Company [Member]
Commitments Contingencies And Guarantees [Line Items]
Contractual obligations		$ 12,700,000	$ 12,700,000
Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Expiration period maximum			1 year
Subsequent Event [Member] | Jefferies [Member]
Commitments Contingencies And Guarantees [Line Items]
Contractual obligations for the remainder of fiscal 2017				$ 2,500,000
Contractual obligation for fiscal 2018				11,900,000
Contractual obligation due annually from 2019 to 2028				$ 15,700,000