Loans To And Investments In Associated Companies (Jefferies LoanCore) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Investments In Associated Companies [Line Items]
Securities purchased under agreements to resell			$ 4,345,461	$ 3,862,488
Payables to brokers, dealers and clearing organizations			2,332,400	3,290,400
Jefferies LoanCore [Member]
Investments In Associated Companies [Line Items]
Aggregate equity commitment			400,000	400,000
Funded equity commitments			141,000	70,100
Equity commitment			$ 194,000	$ 194,000
Percentage of voting interest			48.50%	48.50%
Securities purchased under agreements to resell				$ 68,100
Jefferies [Member]
Investments In Associated Companies [Line Items]
Interest income and fees related to master repurchase agreement	$ 2,300	$ 5,100
Payables to brokers, dealers and clearing organizations			$ 4,800	$ 8,300