Loans to and Investments In Associated Companies (Jefferies Finance) (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investments In Associated Companies [Line Items]
Other	$ 1,506,986,000		$ 1,506,986,000		$ 1,661,777,000
Payables, expense accruals and other liabilities	6,680,224,000		6,680,224,000		7,167,666,000
Financial instruments owned trading assets, at fair value	17,709,245,000		17,709,245,000		16,082,676,000
Jefferies Group
Investments In Associated Companies [Line Items]
Interest income and unfunded commitment fees related to facility commitment	300,000	$ 500,000	2,000,000	$ 3,300,000
Investment banking	71,100,000	104,200,000	282,100,000	243,500,000
Origination fees	12,100,000	0	45,500,000	2,500,000
Placement agent fees	$ 400,000	800,000	$ 3,100,000	4,700,000
Jefferies Finance
Investments In Associated Companies [Line Items]
Equity method investment, ownership percentage	50.00%		50.00%
Equity commitment	$ 750,000,000.0		$ 750,000,000.0
Funded equity commitments	706,500,000		$ 706,500,000
Investment commitment extension			1 year
Termination notice			60 days
Total line of credit facility commitment under joint venture	500,000,000		$ 500,000,000		500,000,000.0
Extension period			1 year
Line of credit facility, commitment of Jefferies, funded	0		$ 0		0
Line of credit facility commitment of Jefferies	250,000,000.0		250,000,000.0		250,000,000.0
Service fee income	13,300,000	$ 7,900,000	48,300,000	$ 37,400,000
Other	36,300,000		36,300,000		34,600,000
Payables, expense accruals and other liabilities	14,100,000		14,100,000		14,100,000
Foreign exchange contracts | Jefferies Finance
Investments In Associated Companies [Line Items]
Payables, expense accruals and other liabilities	$ 200,000		$ 200,000
Financial instruments owned trading assets, at fair value					$ 1,500,000