Fair Value Disclosures (Schedule Of Changes In Fair Value Of Investments Reflected As Principal Transactions) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Changes in fair value of investments reflected as principal transactions	$ (5,688)	$ (363,752)	$ 177,031	$ 10,164
Knight Capital [Member]
Changes in fair value of investments reflected as principal transactions	(5,688)		(5,688)
Mueller [Member]
Changes in fair value of investments reflected as principal transactions		(28,767)		45,548
Jefferies [Member]
Changes in fair value of investments reflected as principal transactions		$ (334,985)	$ 182,719	$ (35,384)