Segment Information (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Unrealized and realized gains and interest income	$ 72,238	$ 175,121	$ 801,473	$ 456,788
Interest expense	30,671	29,144	61,120	57,720
FXCM [Member]
Segment Reporting Information [Line Items]
Unrealized and realized gains and interest income	(112,100)		574,500
National Beef [Member]
Segment Reporting Information [Line Items]
Interest expense	4,100	3,800	8,400	6,900
Parent Company [Member]
Segment Reporting Information [Line Items]
Interest expense	$ 24,300	$ 24,400	$ 48,600	$ 49,500