Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment income and other, net	$ 421,384	$ 41,232	$ 124,095
Income tax expense	(130,858)	(22,753)	101,114
Non-cash gain on Knight Common Stock	0	$ 0	127,972
Knight Capital Group
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-cash gain on Knight Common Stock			128,000
Amounts Reclassified from Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment income and other, net	(497)		(127,972)
Income tax expense	189		48,629
Net of tax	$ (308)		$ (79,343)