Derivative Financial Instruments - Unrealized and Realized Gains (Losses) on Derivative Contracts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in Interest expense of Jefferies	$ 6,033	$ 0	$ 0
Unrealized and realized gains (losses)	(319,318)	(649,789)	(167,626)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized and realized gains (losses)	3,171	(36,559)	(36,792)
Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized and realized gains (losses)	4,376	20,401	36,233
Equity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized and realized gains (losses)	(319,775)	(635,305)	(137,219)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized and realized gains (losses)	(9,049)	(3,339)	(13,625)
Credit contracts
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized and realized gains (losses)	1,959	5,013	(16,223)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in Interest expense of Jefferies	(2,091)	0	0
Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in Interest expense of Jefferies	$ 8,124	$ 0	$ 0