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, 2012	Dec. 31, 2011
Receivables From And Payables To Brokers, Dealers And Clearing Organizations [Abstract]
Deposits paid for securities borrowed	$ 200,734	$ 193,509
Receivable from clearing organizations	14,188	43,642
Securities failed to deliver	22,336	15,485
Receivables from brokers, dealers and clearing organizations, Total	$ 237,258	$ 252,636