Loans to and Investments in Associated Companies - Jefferies LoanCore Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Proceeds from sale of associated companies		$ 173,105	$ 0	$ 0
Securities purchased under agreements to resell		3,689,559	3,862,488
Payables, expense accruals and other liabilities		$ 7,167,666	7,373,708
Jefferies LoanCore
Investment [Line Items]
Percentage of ownership owned	48.50%	48.50%
Proceeds from sale of associated companies	$ 173,100
Period entitled to additional cash consideration	5 years
Interest Income And Fees Related To Master Repurchase Agreement		$ 600	8,400	$ 10,700
Securities purchased under agreements to resell			68,100
Payables, expense accruals and other liabilities			$ 8,300
Jefferies LoanCore | Disposed of by Sale | Jefferies
Investment [Line Items]
Proceeds from sale of associated companies	$ 173,100
Period entitled to additional cash consideration	5 years