Loans to and Investments In Associated Companies (Jefferies Finance) (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investments In Associated Companies [Line Items]
Investment banking		$ 475,702,000	$ 294,930,000	$ 1,235,586,000	$ 778,906,000
Other		1,727,012,000		1,727,012,000		$ 1,635,664,000
Payables, expense accruals and other liabilities		7,104,957,000		7,104,957,000		7,373,708,000
Jefferies [Member]
Investments In Associated Companies [Line Items]
Equity Commitment Contribute	$ 74,800,000
Investment banking		104,200,000	19,000,000	243,500,000	42,100,000
Origination fees				2,500,000	1,600,000
Placement Agent Fees		800,000	1,300,000	4,700,000	1,300,000
Jefferies Finance [Member]
Investments In Associated Companies [Line Items]
Equity commitment	750,000,000.0	750,000,000.0		750,000,000.0
Funded equity commitments		612,100,000		$ 612,100,000
Investment commitment extension				1 year
Termination notice				60 days
Increase of equity commitment	$ 150,000,000.0
Total line of credit facility commitment under joint venture		500,000,000		$ 500,000,000		500,000,000
Extension period				1 year
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
Service fee income		7,900,000	$ 7,200,000	37,400,000	$ 35,800,000
Other		$ 22,500,000		$ 22,500,000
Payables, expense accruals and other liabilities						$ 5,800,000