Receivables From And Payables To Brokers, Dealers And Clearing Organizations (Amounts Receivable From Brokers, Dealers, And Clearing Organizations) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Receivables From And Payables To Brokers, Dealers And Clearing Organizations [Abstract]
Deposits paid for securities borrowed	$ 333,371	$ 153,819
Receivable from clearing organizations	179,442	115,996
Securities failed to deliver	15,374	6,409
Receivables from brokers, dealers and clearing organizations, Total	$ 528,187	$ 276,224