Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 1,401	$ 114,319	$ 0
Other comprehensive income		15,054
Amount reclassified from Accumulated Other Comprehensive Income		(127,972)
Net current-period other comprehensive (loss) income	732	(112,918)	114,319
Ending Balance	2,133	1,401	114,319
Non-cash gain on Knight Common Stock	0	127,972	0
Knight Capital Group [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Non-cash gain on Knight Common Stock		128,000
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	36	114,319	0
Other comprehensive income		13,689
Amount reclassified from Accumulated Other Comprehensive Income		(127,972)
Net current-period other comprehensive (loss) income	316	(114,283)	114,319
Ending Balance	352	36	114,319
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	1,365	0	0
Other comprehensive income		1,365
Amount reclassified from Accumulated Other Comprehensive Income		0
Net current-period other comprehensive (loss) income	416	1,365	0
Ending Balance	$ 1,781	$ 1,365	$ 0